Nb-ti microalloyed cr-mn gear steel and preparation method thereof

Abstract

The present application relates to a kind of Nb-Ti microalloying Cr-Mn gear steel and its preparation method, the chemical composition of the Nb-Ti microalloying Cr-Mn gear steel, by mass percentage, including: C:0.15~0.25%, Si≤0.20, Mn:0.8~1.60%, P:≤0.025%, S:0.010~0.035%, Cr:1.00%~1.80%, Ni:≤0.25%, Cu:≤0.20%, Ti:0.01~0.03%, Nb:0.02~0.07%, B≤0.0005%, Al:0.01~0.04%, [O]:≤12×10 ‑4 %, [N]:0.005~0.010%, [H]:≤1.5×10 ‑4 %, the rest is Fe and inevitable impurities.The Nb-Ti microalloying Cr-Mn gear steel prepared by the application has good hardenability, end hardenability J9=40~45HRc, J15=35~40HRc, which is greatly improved compared with 20CrMnTiH;The gear grain is refined, and the grain size reaches more than 7 levels after carburizing at 960~980 ℃ for 8~10 h, and the strength and toughness and fatigue performance of the gear are significantly improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of alloy gear steel, and relates to a Nb-Ti micro-alloyed Cr-Mn gear steel and a preparation method thereof, which is suitable for manufacturing carburized gears in the fields of locomotives and automobiles. BACKGROUND

[0002] As a basic component, gears are widely used in the automobile and mechanical equipment industries. The annual demand for gear steel in China is nearly 4 million tons. The development of gear steel is related to the quality of the equipment manufacturing industry and the development of the national economy. However, gears need to be carburized for a long time during the manufacturing process, which has a long process cycle and high energy consumption. Therefore, improving the carburizing efficiency of gears is of great significance for reducing carbon emissions, reducing energy consumption and saving costs.

[0003] High-temperature carburizing of gears can shorten the process cycle by more than half, with significant energy-saving and efficiency-increasing effects. 20CrMnTiH gear steel has the lowest price and cost, and is the most widely used in the manufacturing of gearboxes for commercial vehicles in China. However, it has some deficiencies in terms of strength and toughness matching, impact resistance, fatigue life, hardenability, and microstructure uniformity. In addition, the austenite grains of 20CrMnTiH gear steel tend to grow during high-temperature carburizing, which can lead to the formation of coarse grains and mixed crystal structures, resulting in increased quenching deformation and reduced fatigue strength of the gear. Therefore, it is urgent to develop low-cost high-performance high-temperature carburizing gear steel.

[0004] Currently, most carburized gear steels contain 0.02-0.05% Al to refine the grains through the formation of AlN. However, AlN has poor high-temperature stability and can dissolve above 950℃, losing its pinning effect on the grain boundaries. To prevent the austenite grains from growing abnormally after high-temperature carburizing, a small amount of Nb alloy is added to pin the grain boundaries through the formation of Nb(C,N) particles, thereby inhibiting the growth of austenite grains. Although the carburizing temperature can reach 1000℃, Nb(C,N) can also dissolve during the heating process of forging, leading to grain coarsening during heating and affecting the performance of the gear after forging. Ti(C,N) has a higher dissolution temperature than Nb(C,N) and can prevent grain coarsening during high-temperature heating of forging. Therefore, Nb and Ti composite micro-alloying is an effective means to develop high-performance high-temperature carburizing gear steel. SUMMARY

[0005] The present application aims at overcoming the above problems, and provides a Nb-Ti micro-alloyed Cr-Mn gear steel and a preparation method thereof. The Nb-Ti micro-alloyed gear steel prepared by the present application has good hardenability, strength and toughness, and effectively solves the problems of coarse grains and mixed grains in the traditional manganese-chromium gear steel during gear processing, so that the grains are refined, and the deformation of the gear during quenching is reduced.

[0006] To achieve the above-mentioned purposes, the Nb-Ti micro-alloyed Cr-Mn gear steel provided by the present application has the following chemical composition by mass percentage: C: 0.15-0.25%, Si≤0.20, Mn: 0.8-1.60%, P: ≤0.025%, S: 0.010-0.035%, Cr: 1.00%-1.80%, Ni: ≤0.25%, Cu: ≤0.20%, Ti: 0.01-0.03%, Nb: 0.02-0.07%, B≤0.0005%, Al: 0.01-0.04%, [O]: ≤12×10 -4 -4%, [N]: 0.005-0.010%, [H]: ≤1.5×10 -4 -4%, and the rest is Fe and inevitable impurities.

[0007] To ensure the stable hardenability of the gear steel, the further optimized chemical composition of the Nb-Ti micro-alloyed Cr-Mn gear steel is as follows by mass percentage: C: 0.17-0.23%, Si: 0.10-0.20%, Mn: 1.00-1.50%, P: ≤0.025%, S: 0.015-0.025%, Cr: 1.20%-1.50%, Ni: ≤0.25%, Cu: ≤0.02%, Ti: 0.01-0.03%, Nb: 0.03-0.05%, B≤0.0005%, Al: 0.01-0.04%, [O]: ≤12×10 -4 -4%, [N]: 0.005-0.010%, [H]: ≤1.5×10 -4 -4%, and the rest is Fe and inevitable impurities.

[0008] In the present application, the micro-alloying design of the steel is based on low-carbon Cr-Mn gear steel, the hardenability of the material is controlled by adjusting the contents of C, Mn and Cr, Ti(C, N) and Nb(C, N) are dispersedly precipitated by reasonably controlling the contents of Nb, Ti and N and the rolling process, so as to prevent the growth of austenite grains and avoid the generation of liquid-precipitated TiN and its harmful effect on the performance of the gear, the grain coarsening under high-temperature carburizing conditions is prevented, the carburizing efficiency of the high-temperature carburizing heat treatment process is improved, the deformation of the gear during quenching is reduced, and the strength, toughness and fatigue performance of the gear are improved.

[0009] The present invention determines the contents of Ti, Nb, and N in the steel based on thermodynamic calculations and experimental research. To prevent the generation of liquid TiN, the Ti content is controlled within a range of 0.01 to 0.03%, while the N content is controlled within a range of 0.005 to 0.010%. Furthermore, to prevent the adverse effects of excessive Nb (C, N) on plasticity, the Nb content is controlled within a range of 0.02 to 0.07%. During the rolling process, the ingot is first heated to 1230 to 1250°C to fully dissolve the Nb (C, N). The rolling amount and final rolling temperature are then controlled to disperse and precipitate the Nb (C, N). Furthermore, post-rolling cooling is controlled to fully precipitate the Nb (C, N).

[0010] The alloying process design for the steel smelting process in the present invention is designed to ensure steel cleanliness and alloying element yield, and to maximize the effects of different microalloying elements. The order of adding various microalloying alloy materials is rationally designed. During the LF refining process, ferroniobium is added. Before LF tapping, aluminum wire, low-titanium ferro, and high-calcium wire are sequentially fed. After the VD vacuum treatment, manganese nitride wire and sulfur wire are sequentially fed.

[0011] The present invention provides a Nb-Ti microalloyed gear steel and a preparation method thereof based on traditional Cr-Mn gear steel through microalloying design of the steel and alloying process design during its smelting process. The produced gear steel has an austenite grain size of 8 to 9 levels, fine and uniform grains; a J9 hardenability of 40 to 45 HRc, a J15 hardenability of 35 to 40 HRc, a hardenability band controlled to be no greater than 5 HRc, good hardenability and a small fluctuation range; and an impact toughness of ≥90 J. The present invention effectively solves the problems of traditional Cr-Mn gear steel commonly encountered in traditional carburizing heat treatment processes during gear processing, such as coarse grains and mixed crystals.

[0012] The present invention provides a Nb-Ti microalloyed Cr-Mn gear steel. The chemical composition design requirements of the steel are as described above. The preparation method thereof comprises the following steps:

[0013] 1) Smelting

[0014] Electric furnace smelting is performed using molten iron and scrap steel as the raw materials, with the molten iron weight ratio being no less than 60%. The endpoint [C] is controlled to be ≥ 0.07% and [P] ≤ 0.012%, with residual element content meeting design requirements. The tapping temperature is controlled at 1620-1660°C. To ensure full melting of the alloy and create optimal conditions for subsequent refining, the alloy is added to the lower limit of the chemical composition during tapping.

[0015] 2) Refining

[0016] The LF refining process adds 0.40-0.60 Kg / t of ferro-niobium, controls the Al2O3 content of the final slag to be 25%-35%, the basicity of the final slag to be greater than 5.0, and FeO+MnO to be less than or equal to 0.5%, keeps the white slag time during the refining process to be greater than 25 minutes, and sequentially feeds aluminum wire 1.0-2.0 m / t of steel, adds low-titanium ferro-titanium with 25%-35% of titanium content 1.0-1.5 Kg / t of steel, and feeds high-calcium wire 0.5-1.5 m / t of steel before tapping, wherein the calcium content is 93%-96%.

[0017] After refining, the hydrogen content is determined by a hydrogen analyzer, and the hydrogen content is not greater than 1.5 x 10 -4 wt%, the vacuum degree is less than 67 Pa, and the holding time is greater than 15 minutes; after breaking the vacuum of the VD vacuum treatment, manganese nitride wire 0.6-1.5 m / t of steel is sequentially fed, and sulfur wire 0.8-1.5 m / t of steel is fed to alloy the molten steel. The soft argon blowing time of the molten steel is not less than 25 minutes, the molten steel is strictly prohibited from being exposed to the slag surface and large argon volume stirring and cooling during the soft argon blowing, and the molten steel covering agent is added in time during the soft blowing process.

[0018] 3) Pouring

[0019] Double-layer tundish covering agent is used for continuous casting, electromagnetic stirring in the crystallizer and end electromagnetic stirring are adopted. The molten steel temperature superheat degree in the tundish is controlled to be 18-30 ℃, and the casting speed is 0.30-0.40 m / min to ensure the quality of the casting blank.

[0020] 4) Rolling

[0021] The soaking temperature of the billet in the heating furnace is 1230-1250 ℃, the high-temperature heating time is greater than 2.5 h, the total heating time is 4.5-5.5 h, the starting rolling temperature is 1060-1140 ℃, and the final rolling temperature is 880-950 ℃.

[0022] The rolled material is cooled on the cooling bed using a heat preservation cover to ensure that the cooling speed of the steel is controlled to be less than or equal to 25 ℃ / min, and the steel is quickly cooled to the pit after being cooled to 620-670 ℃ on the cooling bed and then slowly cooled to below 200 ℃.

[0023] The Nb-Ti micro-alloyed Cr-Mn gear steel prepared by the application has good hardenability, the hardenability of the Nb-Ti micro-alloyed Cr-Mn gear steel is detected according to GB / T 225, the normalizing temperature of an end-quenching sample is 920±10 DEG C, the quenching temperature is 880 DEG C±5 DEG C, the end-quenching hardenability J9 is 40-45 HRc, and the end-quenching hardenability J15 is 35-40 HRc, which is greatly improved compared with 20CrMnTiH; the gear grain is refined, the gear blank after heat treatment of the Nb-Ti micro-alloyed Cr-Mn gear steel is processed, then is subjected to carburizing treatment at 960 DEG C-980 DEG C, and after carburizing for 8-10 hours and quenching, the grain size is above grade 7, and the strength and toughness and fatigue performance of the gear are significantly improved.

[0024] Compared with the prior art, the application has the following advantages:

[0025] 1) The application is based on Cr-Mn gear steel, the content range of micro-alloying elements Nb, Ti and N in the steel is controlled, and the controlled rolling process is used, so that Ti(C, N) and Nb(C, N) are dispersedly precipitated to prevent the growth of austenite grains, and the generation of liquid-precipitated TiN and its harmful effect on the performance of the gear are avoided, the gear steel produced has fine and uniform grains, the J9 hardenability is 40-45 HRc, the J15 hardenability is 35-40 HRc, and the hardenability band control is not more than 5 HRc.

[0026] 2) The application provides a micro-alloyed gear steel and a preparation method thereof on the basis of the traditional Cr-Mn gear steel through the micro-alloying of the steel and the design of the metallurgical process, the gear steel prepared is suitable for high-temperature carburizing heat treatment process, through the micro-alloying design, the grain coarsening under the high-temperature carburizing condition is prevented, the grains of the steel are refined, the carburizing efficiency under the high-temperature carburizing heat treatment process condition is improved, the quenching deformation of the gear is reduced, and the strength and toughness and fatigue performance of the gear are improved. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is the microstructure diagram of the Nb-Ti micro-alloyed Cr-Mn gear steel embodiment 1 of the application. DETAILED DESCRIPTION

[0028] Any feature disclosed in this specification, unless stated otherwise, can be replaced by any equivalent or similar feature. Unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The disclosure of any aspect in the specification should be considered in the context of the entire specification and not confined to that aspect alone.

[0029] The application will be further described below with reference to specific embodiments.

[0030] Embodiment 1 to embodiment 3

[0031] The application provides a Nb-Ti micro-alloyed Cr-Mn gear steel, which is smelted by using an UHP (ultra high power) electric arc furnace, an LF furnace outside refining and a vacuum degassing treatment process, and is produced into a steel material by continuous casting a round billet. The embodiment is a production process of a Φ500 mm round billet and a Φ130 mm round steel, which specifically illustrates how to implement the application.

[0032] The production process is as follows:

[0033] 1) Smelting

[0034] The electric furnace smelting is adopted, the smelting raw material is molten iron + scrap steel, the proportion of molten iron is not less than 60%, the end point [C] is controlled to be 0.09% to 0.10%, the [P] is controlled to be less than or equal to 0.009%, the residual element content meets the design requirement, the tapping temperature is controlled to be 1630 to 1650 DEG C, in order to ensure that the alloy is fully melted and to create good conditions for the subsequent refining process, the alloy amount is added to the lower limit of the chemical composition as far as possible when the electric furnace is tapped.

[0035] 2) Refining

[0036] The niobium iron of 0.40 to 0.60 Kg / t steel is added in the LF refining process, the Al2O3 content of the final slag is controlled to be 25% to 35%, the final slag basicity is greater than 5.0, the FeO+MnO is less than or equal to 0.5%, the white slag time in the refining process is kept to be greater than 25 minutes, the aluminum wire of 1.0 to 2.0 m / t steel is sequentially fed before the LF tapping, the low titanium iron with the titanium content of 25% to 35% of 1.0 to 1.5 Kg / t steel is added, and the high calcium wire of 0.5 to 1.5 m / t steel is fed.

[0037] After the refining, the vacuum treatment is carried out, the hydrogen content is determined by using a hydrogen determination instrument, the hydrogen content is not greater than 1.5*10 -4 wt%, the vacuum degree is 40 Pa, the holding time is 16 to 17 minutes; after the VD vacuum treatment is broken, the manganese nitriding wire of 0.6 to 1.5 m / t steel and the sulfur wire of 0.8 to 1.5 m / t steel are sequentially fed to carry out the molten steel alloying. The soft argon blowing time of the molten steel is 27 to 30 minutes, the molten steel and the large argon amount stirring and cooling are strictly prohibited when the soft argon blowing, the molten steel covering agent is added in time during the soft blowing process.

[0038] 3) Pouring

[0039] The double-layer tundish covering agent is used in the continuous casting, the crystallizer electromagnetic stirring and the terminal electromagnetic stirring are adopted. The molten steel temperature overheat degree in the tundish is controlled to be 19 to 25 DEG C, and the pulling speed is 0.35 m / min, so as to ensure the billet quality.

[0040] 4) Rolling

[0041] The soaking temperature of the billet in the heating furnace is 1235-1248 DEG C, the high-temperature section heating time is 2.6-2.7 h, the total heating time is 4.8-5.2 h, the open rolling temperature is 1110-1136 DEG C, and the final rolling temperature is 905-921 DEG C;

[0042] The rolled material is covered with an insulation cover on the cooling bed to ensure that the cooling speed of the steel is controlled at ≤25 DEG C / min, and the steel is cooled to 620-670 DEG C and then quickly lowered to the pit to be slowly cooled to below 200 DEG C.

[0043] The Nb-Ti micro-alloyed gear steel prepared by the method has good hardenability, strength and toughness, and fatigue performance, effectively solves the problems of coarse grains and mixed grains in the traditional manganese-chromium gear steel during gear processing, refines the grains, and reduces the deformation during gear quenching.

[0044] Table 1 is the chemical composition of the example, Table 2 is the smelting process control parameter of the example, Table 3 is the continuous casting and rolling process control parameter of the example, Table 4 is the grain size test of the example, Table 5 is the performance index test of the hot rolled round steel of the example, and Table 6 is the end quenching property test of the hot rolled round steel of the example.

[0045] Table 1 Chemical composition (wt%)

[0046]

[0047]

[0048] Table 2 Smelting process control parameters

[0049]

[0050] Table 3 Continuous casting and rolling process control parameters

[0051]

[0052] Table 4 Grain size test

[0053]

[0054]

[0055] Table 5 Performance index test of hot rolled round steel

[0056]

[0057] Table 6 End quenching property test of hot rolled round steel

[0058]

[0059] Finally, it should be noted that the above examples are merely used to illustrate the technical solutions of the present application but not to limit. Although the present application is explained in detail with reference to the examples, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the spirit and scope of the present application, and all of them should be covered in the scope of the claims of the present application.

Claims

1. A Nb-Ti microalloyed Cr-Mn gear steel, characterized in that: The chemical composition of the Nb-Ti microalloyed Cr-Mn gear steel, calculated by mass percentage, includes: C: 0.15-0.25%, Si≤0.20%, Mn: 0.8-1.60%, P: ≤0.025%, S: 0.010-0.035%, Cr: 1.00%-1.80%, Ni: ≤0.25%, Cu: ≤0.20%, Ti: 0.01-0.03%, Nb: 0.02-0.07%, B≤0.0005%, Al: 0.01-0.04%, [O]: ≤12×10 -4 %, [N]: 0.005~0.010%, [H]: ≤1.5×10 -4 %; the rest is Fe and unavoidable impurities; The method for preparing the Nb-Ti microalloyed Cr-Mn gear steel comprises the following steps: 1) Smelting: Control the tapping temperature at 1620-1660°C; 2) Refining: Ferroniobium is added during the LF refining process, and aluminum wire, low-titanium ferro and high-calcium wire are fed in sequence before LF tapping; After refining, vacuum treatment is carried out. After breaking the vacuum during VD vacuum treatment, the manganese nitride wire and the sulfur wire are fed in sequence to alloy the molten steel. Control the Al2O3 content of the final slag to 25% to 35%, the basicity of the final slag to be greater than 5.0, FeO+MnO≤0.5%, and the white slag time to be maintained for more than 25 minutes during the refining process; The addition amount of ferroniobium is 0.40-0.60 kg / t steel, the addition amount of aluminum wire is 1.0-2.0 m / t steel, the addition amount of low-titanium ferro with a titanium content of 25%-35% is 1.0-1.5 kg / t steel, and the addition amount of high-calcium wire with a calcium content of 93%-96% is 0.5-1.5 m / t steel; After refining, vacuum treatment is performed, and the hydrogen content is not more than 1.5×10 -4 wt%, vacuum degree is less than 67Pa, holding time is greater than 15 minutes, the amount of manganese nitride wire added is 0.6-1.5m3 / t steel, and the amount of sulfur wire added is 0.8-1.5m3 / t steel; 3) Pouring: Use double-layer tundish covering agent for continuous casting, control the tundish molten steel temperature to be superheated at 18-30°C, and the casting speed to be 0.30-0.40m / min; 4) Rolling: The starting rolling temperature is 1060~1140℃, and the finishing rolling temperature is 880~950℃; 5) Slow cooling; The rolled material is covered with a heat preservation cover on the cooling bed, and the cooling rate of the steel is controlled at ≤25℃ / min. When the steel cools to 620-670℃, it is quickly lowered from the cooling bed and slowly cooled to below 200℃ in the pit. The hardenability of the Nb-Ti microalloyed Cr-Mn gear steel is as follows: the normalizing temperature of the end quenching sample is 920±10°C, the quenching temperature is 880°C±5°C, J9=40-45HRC, and J15=35-40HRC; The gear blank of the Nb-Ti microalloyed Cr-Mn gear steel after heat treatment is processed and then carburized at 960-980° C. for 8-10 hours and quenched to have a grain size of more than level 7.

2. The Nb-Ti microalloyed Cr-Mn gear steel according to claim 1, characterized in that: The chemical composition of the Nb-Ti microalloyed Cr-Mn gear steel is, by mass percentage, as follows: C: 0.17-0.23%, Si: 0.10-0.20%, Mn: 1.00-1.50%, P: ≤0.025%, S: 0.015-0.025%, Cr: 1.20-1.50%, Ni: ≤0.25%, Cu: ≤0.02%, Ti: 0.01-0.03%, Nb: 0.03-0.05%, B ≤0.0005%, Al: 0.01-0.04%, [O]: ≤12×10 -4 %, [N]: 0.005~0.010%, [H]: ≤1.5×10 -4 %, and the rest are Fe and inevitable impurities.

3. The Nb-Ti microalloyed Cr-Mn gear steel according to claim 1, characterized in that: In step 1), electric furnace smelting is adopted, the smelting raw materials are molten iron + scrap steel, the weight proportion of molten iron is not less than 60%, and the end point [C] is controlled to be ≥ 0.07% and [P] ≤ 0.012%.

4. The Nb-Ti microalloyed Cr-Mn gear steel according to claim 1, characterized in that: In step 4), the steel billet is heated at a soaking temperature of 1230-1250° C. in the heating furnace, the heating time in the high temperature section is greater than 2.5 h, and the total heating time is 4.5-5.5 h.

Images