阅读说明：本技术 一种余热淬火-自配分制备1380MPa级别贝氏体钢轨的方法 (Method for preparing 1380 MPa-level bainite steel rail through waste heat quenching-self-distribution ) 是由 高古辉 白秉哲 桂晓露 张敏 谭谆礼 翁宇庆 于 2019-09-12 设计创作，主要内容包括：本发明涉一种余热淬火-自配分制备1380MPa级别贝氏体钢轨的方法,以低碳Mn-Si-Cr为主要合金元素,添加Ni、Mo、N、V、Nb等元素,其余为Fe,将经冶炼、铸造、轧制后的余热钢轨,进行余热淬火,使钢轨踏面以0.2～20℃/秒的冷速加速冷却至一定温度之后,使钢轨踏面自返温至马氏体转变开始温度之下的温度进行配分,之后再将钢轨冷却至室温,经回火后获得1380MPa级别贝氏体钢轨。本发明所得贝氏体钢轨的抗拉强度为1380～1480MPa,屈服强度为980～1300MPa,延伸率大于12％,冲击韧性大于100J,耐磨性显著改善。(The invention relates to a method for preparing 1380 MPa-level bainite steel rails by waste heat quenching-self-distribution, which comprises the steps of taking low-carbon Mn-Si-Cr as a main alloy element, adding elements such as Ni, Mo, N, V, Nb and the like, and the balance Fe, carrying out waste heat quenching on the waste heat steel rails after smelting, casting and rolling, carrying out waste heat quenching on a steel rail tread, carrying out accelerated cooling at a cooling speed of 0.2-20 ℃/s to a certain temperature, distributing the steel rail tread from the temperature return to the temperature below the martensite transformation starting temperature, then cooling the steel rails to room temperature, and tempering to obtain 1380 MPa-level bainite steel rails. The bainite steel rail obtained by the method has the tensile strength of 1380-1480 MPa, the yield strength of 980-1300 MPa, the elongation of more than 12 percent, the impact toughness of more than 100J and obviously improved wear resistance.)

1. A method for preparing 1380MPa bainite steel rails by waste heat quenching-self-distribution is characterized by comprising the following steps:

1) smelting and casting bainite steel containing C, Mn, Si, Cr, Ni, Mo, N, V, Nb and Fe to prepare a square billet;

2) the square billet is heated again and rolled to obtain a waste heat steel rail;

3) quenching the waste heat steel rail, accelerating the cooling of the tread of the steel rail, distributing the tread of the steel rail from the temperature return to the temperature below the martensite transformation starting temperature, and then cooling the steel rail to the room temperature;

4) and tempering the steel rail to obtain a 1380MPa bainite steel rail.

2. The method for preparing 1380MPa bainite steel rails through waste heat quenching and self-distribution according to claim 1, wherein the content of each alloy element of the bainite steel in the step 1) comprises the following components in percentage by mass:

c: 0.12-0.22 wt.%; mn: 2.0-2.4 wt.%; si: 0.6-1.2 wt.%; cr: 0.5-1.0 wt.%; ni: 0.2-0.6 wt.%; mo: 0.2-0.4 wt.%; n: 0.001-0.005 wt.%; v: 0-0.2 wt.%; nb: 0-0.04 wt.%; the balance being Fe and unavoidable impurities.

3. The method for preparing 1380MPa grade bainite steel rails through waste heat quenching-self distribution according to claim 1, wherein the waste heat steel rails in the step 2) are as follows: and (3) rolling the square billet, wherein the residual heat temperature of the rolled steel rail tread is 850-1000 ℃.

4. The method for preparing 1380MPa grade bainite steel rails through waste heat quenching-self distribution according to claim 1, wherein the cooling speed of accelerated cooling of the steel rail tread in the step 3) is 0.2-20 ℃/s.

5. The method for preparing 1380MPa grade bainite steel rail by waste heat quenching-self distribution according to claim 1, wherein the temperature to which the steel rail tread is accelerated and cooled in step 3) is (150-280 ℃) X (75 ÷ X) according to the specification of the steel rail, wherein X is kilogram per meter of the steel rail.

6. The method for preparing 1380MPa bainite steel rail by waste heat quenching-self distribution according to claim 1, wherein the step 3) is to distribute the temperature of the tread of the steel rail from the temperature return to the temperature 10-80 ℃ below the martensite transformation starting temperature.

7. The method for preparing 1380MPa grade bainite steel rails through waste heat quenching-self distribution according to claim 1, wherein the steel rail tempering in the step 4) is as follows: and (3) preserving the temperature of the steel rail at 260-350 ℃ for 10-60 hours, and then naturally cooling the steel rail to room temperature.

8. The method for preparing 1380MPa bainite steel rail by waste heat quenching-self distribution according to claim 1, wherein the 1380MPa bainite steel rail in the step 4) is: according to TB/T2344-: a tensile sample is taken from the technical condition of ordering the steel rail of 43 kg/m-75 kg/m, and the tensile strength is 1380-1480 MPa.

Technical Field

The invention relates to the technical field of steel for railways, in particular to a method for preparing 1380MPa bainite steel rails by waste heat quenching-self-distribution.

Background

At present, the transportation of bulk and long-distance goods in China is mainly borne by freight railways, and the freight capacity of the railways directly influences the development of national economy in China. The heavy haul railway is the most effective way for improving the freight capacity and is also an important direction for railway development in China. The heavy haul railway has the characteristics of large axle weight, large transportation capacity, high departure density and the like, and provides more rigorous requirements on the strength, toughness, wear resistance and fatigue resistance of steel rail materials, the existing steel rail performance in China cannot be completely met, the heavy haul railway becomes a bottleneck link restricting the development of the heavy haul railway in China, and the research and development of a new generation of heavy haul steel rails is imperative.

Compared with the common pearlite steel rail, the bainite steel rail has higher obdurability, better contact fatigue resistance and weldability. Under the background, many scientists are concerned about the development and production of bainite steel rails. For example, Beijing Temetallurgical industry and trade, Inc. has proposed "Bainite steel and bainite steel rail for curve and heavy-duty rail and its production method" (publication No. CN 101921971A), but because of adopting the air cooling and cooling with the cooling rate of 10-20 ℃/minute, the intensity of the obtained rail is low, only 1200-1300 MPa, can not meet the heavy-duty railway construction that develops day by day. "Bainite structural steel rail having excellent fatigue resistance and method for producing the same" (publication No. CN 1978690A) are proposed by saddle Steel products, but "natural cooling in air after hot rolling" inevitably causes influence of seasonal change on cooling rate, resulting in non-uniform performance and structure. The iron and steel research institute proposes 'an alloy system and a heat treatment method of bainite steel rails thereof' (publication number is CN 105385938A), but because the steel rails need to be subjected to re-austenitizing and normalizing treatment, waste heat treatment is not adopted, and the method is not beneficial to energy conservation and emission reduction. The Panzhihua steel research institute company Limited of Panzhihua group proposed "a high strength bainite steel rail and its production method" (publication number is CN 104087852A); although the waste heat treatment process is adopted, the self-distribution and tempering steps are not adopted, and the martensite is brittle and is not beneficial to improving the service performance of the steel rail.

The bainite/martensite/retained austenite/carbide complex phase structure is an ideal structure of a bainite steel rail with high strength grade (more than 1380MPa), and the necessary martensite structure and carbide are beneficial to improving the strength and the wear resistance of the bainite steel rail; the carbon distribution is utilized to promote the diffusion of carbon in martensite into austenite, which is beneficial to improving the toughness of the martensite and increasing the stability of the complex phase structure. For example, Beijing university of transportation proposes "a BQ & P heat treatment process of carbide-free bainite/martensite duplex phase steel" (publication number is CN 106893832A), and introduces a partitioning process to improve the comprehensive mechanical properties of the duplex phase steel; however, in the patent, reheating is needed in the distribution stage, which is not beneficial to energy conservation and emission reduction on one hand; on the other hand, reheating is difficult to achieve for a hundred meter rail. Therefore, the invention is necessary to invent a method suitable for industrial production of the Baimi fixed-length bainitic steel rail.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a method for preparing 1380MPa bainite steel rails by waste heat quenching-self-distribution, which solves the problems of poor toughness, insufficient wear resistance and insufficient fatigue resistance of the steel rails for the current heavy haul railway and prolongs the service life of the heavy haul steel rails; on the other hand, the method is suitable for industrial production of hundred-meter fixed-length heavy-load steel rails, and is beneficial to energy conservation and emission reduction.

The technical scheme for solving the problems is as follows: a method for preparing 1380MPa bainite steel rail by waste heat quenching-self-distribution is characterized by comprising the following steps:

1) smelting and casting bainite steel containing C, Mn, Si, Cr, Ni, Mo, N, V, Nb and Fe to prepare a square billet, and slowly cooling to room temperature;

2) the square billet is heated again and rolled to obtain a waste heat steel rail;

3) quenching the waste heat steel rail, after the tread of the steel rail is cooled to a certain temperature in an accelerated way, distributing the tread of the steel rail from the temperature return to the temperature below the martensite transformation starting temperature, and then cooling the steel rail to the room temperature;

4) and tempering the steel rail to obtain a 1380MPa bainite steel rail.

Preferably, the content of each alloying element of the bainite steel in the step 1) comprises the following components in percentage by mass: c: 0.12-0.22 wt.%; mn: 2.0-2.4 wt.%; si: 0.6-1.2 wt.%; cr: 0.5-1.0 wt.%; ni: 0.2-0.6 wt.%; mo: 0.2-0.4 wt.%; n: 0.001-0.005 wt.%; v: 0-0.2 wt.%; nb: 0-0.04 wt.%; the balance being Fe and unavoidable impurities.

In the invention, considering that the heavy-duty steel rail has large cross section and is a special-shaped cross section, and the cooling speed of the tread position of the rail head is difficult to be consistent with that of other positions, Mn-Si-Cr is adopted as a main alloy to inhibit high-temperature ferrite transformation and promote bainite transformation, elements such as Ni-Mo and the like are supplemented to reduce the martensite transformation starting temperature and expand the bainite transformation interval, so that the steel rail can obtain a bainite/martensite complex phase structure within the cooling speed range of 2-20 ℃/s. The C content adopts low carbon, and high strength and toughness and weldability are considered. Meanwhile, V, Nb microalloying is adopted in the invention, so that fine precipitates are formed, and the wear resistance of the steel rail is improved; in order to make V and Nb function as much as possible, the content of N is controlled to form an appropriate amount of VN precipitation.

Preferentially, the waste heat steel rail in the step 2) is as follows: and (3) rolling the square billet, wherein the residual heat temperature of the rolled steel rail tread is 850-1000 ℃. Proper rolling temperature is favorable for obtaining uniform and fine prior austenite grains, and if the rolling temperature is too high, the grain refinement is not favorable; if the concentration is too low, incomplete recrystallization may occur, resulting in mixed crystals.

Preferentially, the cooling speed of the accelerated cooling of the steel rail tread in the step 3) is 0.2-20 ℃/second. Within the cooling speed range, a bainite/martensite multiphase structure can be obtained, if the cooling speed is too low, coarse granular bainite can appear at the center of the steel rail, and if the cooling speed is too high, a large amount of martensite can appear on the surface of the steel rail, which is not beneficial to improving the performance.

Preferably, the accelerated cooling of the rail tread in step 3) to a certain temperature is (150-280 ℃) X (75 ÷ X) according to the rail specification, wherein X is the number of kilograms per meter of rail. Heavy-duty steel rails are of special-shaped large-section structures, and the temperature fields of the cross sections of the steel rails with different specifications are not completely the same in the cooling process, so that the final cooling temperature needs to be determined according to the specifications of the steel rails. The formula of the cooling temperature provided by the invention is that X is the specification of the steel rail, for example, 75kg/m and 60kg/m are the most common in heavy-duty steel rails; according to the formula, the cooling temperature is 150-280 ℃ for a 75kg/m steel rail, and is 187.5-350 ℃ for a 60kg/m steel rail, wherein the cooling temperature is (150-280 ℃) multiplied by (75-60). Meanwhile, the cooling temperature cannot be too high, otherwise, the obtained martensite is low in content, and the strength is lower than 1380 MPa; the cooling temperature cannot be too low, otherwise, the obtained martensite has higher content and the toughness is reduced.

Preferably, step 3) is performed by distributing the rail tread from the temperature return to a temperature 10 to 80 ℃ below the martensite start temperature. The invention adopts the self-temperature return to increase the temperature of the tread of the steel rail without additional heating, is suitable for the industrial production of hundreds of meters of fixed-length heavy-load steel rails, and is beneficial to energy conservation and emission reduction. The distribution is adopted to promote the diffusion of carbon in the martensite into austenite to form carbon-poor martensite, thereby being beneficial to the improvement of the toughness and the fatigue performance; on the other hand, by selecting the distribution at the martensite start temperature (Ms) or lower, the bainite structure formed in the distribution stage can be refined. Meanwhile, the temperature below Ms should not be too high, otherwise fine bainite is difficult to form; it should not be too low, otherwise the carbon diffusion rate is slowed, which is detrimental to the formation of carbon depleted martensite. The Ms point is determined by reference to YB/T5128-1993 method for determining continuous cooling transformation curve of steel (expansion method).

Preferably, the tempering of the rail in step 4) is: and (3) preserving the temperature of the steel rail at 260-350 ℃ for 10-60 hours, and then naturally cooling the steel rail to room temperature. The tempering adopted in the invention aims to further promote the diffusion of carbon in martensite and bainitic ferrite into the residual austenite, reduce the internal stress of martensite and improve the stability of the residual austenite; secondly, fine carbide of micro-alloying elements is formed in the tempering process, and the wear resistance of the steel rail is improved.

Preferentially, the 1380MPa grade bainite rail obtained in step 4) is according to TB/T2344-2012: a tensile sample is taken from the technical condition of ordering the steel rail of 43 kg/m-75 kg/m, and the tensile strength is more than 1380MPa, particularly 1380-1480 MPa. The tensile strength is not too low, otherwise, the wear resistance of the steel rail is insufficient; and the service stability of the steel rail is not deteriorated.

The invention has the advantages that:

according to the invention, the steel rail is composed of accurate alloy elements, the heavy-duty steel rail profile is considered, the temperature, cooling speed and time parameters of waste heat quenching, self-distribution and tempering are accurately controlled, a complex phase structure with reasonable bainite, martensite, residual austenite and carbonitride proportions is accurately obtained, and the tensile strength of the steel rail is controlled to be 1380-1480 MPa; the wear resistance and the fatigue resistance of the heavy-duty bainite steel rail can be effectively improved, and the service life of the steel rail is prolonged; the mechanical property of the steel rail can reach: the tensile strength is 1380-1480 MPa, the yield strength is 980-1300 MPa, the elongation is more than 12%, and the impact toughness is more than 100J; according to the standard of GB/T1244.1-90 metal wear test method MM type wear test, compared with a heat treatment type U75V pearlite steel rail, the weight loss rate of the 1380MPa bainite steel rail obtained by the invention is reduced by more than 1 time, namely the wear resistance is improved by more than 1 time.

Meanwhile, the invention adopts the processes of waste heat quenching and self-distribution, is suitable for the industrial production of the hundred-meter fixed-length heavy-load steel rail on the one hand, shortens the process flow, avoids reheating, saves energy and resources, and meets the strategic requirements of energy conservation and emission reduction in China.

Drawings

Fig. 1 shows steps 2), 3) and 4) of a method for preparing 1380MPa bainite steel rails by waste heat quenching-self-distribution.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings of the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

Table 1 shows the alloy components, mass percent and Ms temperature (DEG C) of heavy-duty bainite steel rails of different specifications