Steel for heavy truck bridge gear and preparation method thereof

Abstract

The invention provides steel for a heavy truck axle gear and a preparation method of the steel. The preparation method comprises the following steps: firstly, applying reasonable Al control, N control and Al / N component design (Al / N is more than or equal to 2.5 and less than or equal to 4.0) in initial raw materials and performing accurate control; and the processes of primary smelting, LF refining, degassing treatment, pouring and rolling are matched to obtain the steel for the axle gear of the heavy truck. Through testing, the annealed banded structure of the obtained steel is smaller than or equal to 2.0 grade, the grain size is larger than or equal to 8.0 grade, and the steel can completely meet the use standard requirements for tooth profile precision and contact area precision after heat treatment of the heavy truck axle gear.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to steel for heavy truck axle gears and a preparation method thereof. Background Art

[0002] Axle bevel gears are key transmission components of automobiles, which can transmit motion and power between intersecting shafts. They are generally required to have performance characteristics such as strong load-bearing capacity, high transmission efficiency, good stability, and low noise. The above performance characteristics are even more important for heavy trucks with high loads and continuous rotation in complex road conditions.

[0003] After years of development, my country's heavy-duty truck axle gear market has risen rapidly and has a certain market scale and competitiveness. At present, the commonly used steel grades for heavy-duty truck axle gears are low-carbon, low-alloy carburizing steels such as the Cr-Mo series, Cr-Ni series, and Cr-Ni-Mo series. Taking into account the impact of cost and quality control, the Cr-Mo series has the largest amount of steel. Traditional heavy-duty truck axle gear steel is generally manufactured using the process of cutting-heating-forging-normalizing-machining-tooth cutting-carburizing heat treatment. However, due to the unreasonable composition design and production process design of the Cr-Mo series gear steel, it usually leads to a steel grain size of ≤7.0 and an annealed banded structure of ≥2.5. The grain size and annealed banded structure are not ideally controlled, which will result in the heavy-duty truck axle gears processed by this series of gear steels being unable to meet the standard requirements for the use of heavy-duty truck axle gears after carburizing heat treatment. Summary of the invention

[0004] In view of this, the object of the present invention is to provide a heavy truck axle gear steel and a preparation method thereof. The heavy truck axle gear steel can well meet the use standard requirements of the tooth shape accuracy and contact area accuracy of the heavy truck axle gear after heat treatment.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In the first aspect, the present invention provides a steel for a heavy truck bridge gear, which comprises, by mass fraction, 0.20-0.25% C, 0.17-0.35% Si, 0.75-0.95% Mn, 1.00-1.30% Cr, 0.025-0.045% Al, 0.35-0.45% Mo, 0.015-0.030% S, P≤0.012%, Ca≤0.0008%, Cu≤0.20%, O≤10×10 -4 %,90×10 -4 %≤N≤130×10 -4 % and H≤1.5×10 -4 %, the balance is Fe and impurities;

[0007] Among them, 2.5≤Al / N≤4.0.

[0008] Preferably, the heavy truck bridge gear steel comprises, by mass fraction, 0.22-0.25% C, 0.17-0.30% Si, 0.80-0.95% Mn, 1.00-1.25% Cr, 0.025-0.040% Al, 0.35-0.40% Mo, 0.020-0.030% S, P≤0.011%, Ca≤0.0007%, Cu≤0.18%, O≤9×10 -4 %,85×10 -4 %≤N≤130×10 -4 % and H≤1.2×10 -4 %, the balance is Fe and impurities.

[0009] In a second aspect, the present invention provides a method for preparing the above-mentioned heavy truck bridge gear steel, characterized in that it comprises the following steps:

[0010] The steel for heavy truck axle gears is obtained by primary refining, LF refining, degassing treatment, casting and rolling.

[0011] Preferably, the pouring speed is 0.20-0.40 m / min.

[0012] Preferably, during the pouring process, the superheat of the molten steel is 18-30°C.

[0013] Preferably, during the pouring process, the secondary cooling water volume is 0.12-0.18 L / Kg.

[0014] Preferably, the casting is performed by combining crystallization electric stirring and terminal electric stirring.

[0015] Preferably, the current intensity of the crystallization electric stirring is 100-200 A, and the frequency is 1.0-1.5 Hz.

[0016] Preferably, the current intensity of the terminal electric stirring is 250~350 A, and the frequency is 6.0~7.0 Hz.

[0017] Preferably, the rolling comprises: heating the cast billet after pouring to obtain a hot cast billet, and rolling and cooling the hot cast billet.

[0018] Preferably, the heating includes a preheating section, a first heating section, a second heating section and a soaking section.

[0019] Preferably, the temperature of the preheating section is 500-1000°C.

[0020] Preferably, the temperature of the heating stage is 1050-1200°C.

[0021] Preferably, the temperature of the second heating stage is 1200-1280°C.

[0022] Preferably, the temperature of the soaking section is 1190-1250°C.

[0023] Preferably, the total heating time is 7.0 to 9.0 h.

[0024] Preferably, the starting rolling temperature is 1110-1170°C, and the final rolling temperature is 920-1020°C.

[0025] Preferably, the rolling reduction ratio is ≥9.

[0026] Preferably, the cast ingot is subjected to a slow cooling treatment before rolling.

[0027] Preferably, the entry temperature of the slow cooling treatment is greater than 500°C, and the exit temperature is less than 150°C.

[0028] Preferably, the slow cooling treatment time is not less than 48 h.

[0029] Preferably, the primary smelting is carried out in an electric furnace or a converter.

[0030] Preferably, the degassing treatment is VD degassing or RH degassing.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The invention discloses a heavy truck bridge gear steel. First, the steel is obtained by using reasonable Al, N and Al / N composition design and precise control in the initial raw materials; and by using primary refining, LF refining, degassing, casting and rolling processes. After testing, the annealed banded structure of the obtained steel is ≤2.0 grade, and the grain size is ≥8.0 grade, which can fully meet the standard requirements for the tooth shape accuracy and contact area accuracy of heavy truck bridge gears after heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the isothermal annealed banded structure diagram of the steel material in Example 2 (100 μm);

[0034] Figure 2 This is the grain size diagram of the steel in Example 2 (100 μm). DETAILED DESCRIPTION

[0035] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] The present invention provides a steel for a heavy truck bridge gear, which comprises, by mass fraction, 0.20-0.25% of C, 0.17-0.35% of Si, 0.75-0.95% of Mn, 1.00-1.30% of Cr, 0.025-0.045% of Al, 0.35-0.45% of Mo, 0.015-0.030% of S, P≤0.012%, Ca≤0.0008%, Cu≤0.20%, O≤10×10 -4 %,90×10 -4 %≤N≤130×10 -4 % and H≤1.5×10 -4 %, the balance is Fe and impurities.

[0037] The present invention designs the chemical composition of the steel for heavy truck axle gears, comprehensively considers product quality and process cost control, limits the Al content in the steel to 0.025-0.045%, and limits the N content to 90×10 -4 %≤N≤130×10 -4 %, and satisfy 2.5≤Al / N≤4.0, preferably 2.8≤Al / N≤3.5. The reason is that if the Al content is too high, it is easy to cause secondary oxidation, forming Al2O3 inclusions, affecting the performance of steel; on the contrary, if the N content is too high, it is easy to cause surface quality defects in steel; at the same time, if the content of each component of Al and N is too low, the expected grain refinement effect will not be achieved.

[0038] In addition, in order to meet the comprehensive performance of steel, the present invention designs C: 0.20~0.25%, Si: 0.17~0.35%, and adopts Mn, Cr, and Mo alloying at the same time, designs Mn: 0.75~0.95%, Cr: 1.00~1.30%, and Mo: 0.35~0.45%; in order to reduce the content of spherical non-deformable calcium aluminate inclusions in steel and increase the service life of the product, the Ca content is controlled to be ≤0.0008%. In order to improve the easy cutting performance of steel, S is designed to be 0.015~0.030%. In order to improve the purity of steel to ensure the fatigue strength of steel and the comprehensive performance of steel, the gas and harmful element content of steel is strictly controlled, and O is designed to be ≤10×10 -4 %,H≤1.5×10 -4 %,Cu≤0.20%.

[0039] The present invention also provides a method for preparing the above-mentioned steel for heavy truck bridge gears, comprising the following steps:

[0040] The steel for heavy truck axle gears is obtained by primary refining, LF refining, degassing treatment, casting and rolling.

[0041] In the present invention, firstly, the raw materials meeting the ratio requirements are obtained according to the component design.

[0042] In the primary refining furnace, the steel materials (molten iron, scrap steel) and auxiliary materials (lime, etc.) that meet the ratio requirements are smelted into primary molten steel with C, P composition and temperature that meet the requirements. Then, deoxidation and alloying materials (pre-deoxidation materials that meet the requirements, various alloy materials) and slag (refined synthetic slag) are added during the steelmaking process. The refining process adjusts and controls the various component elements of the molten steel, removes deoxidation inclusions and controls the temperature. The degassing treatment is mainly to control the removal of [H] to prevent hydrogen-induced cracks.

[0043] According to the present invention, after the raw materials are obtained, they are firstly subjected to primary refining. The primary refining can be carried out by electric furnace primary refining or converter primary refining. The present invention has no particular limitation on the operation of primary refining, and it can be carried out according to means well known to those skilled in the art.

[0044] After the initial refining, the molten steel is refined.

[0045] In the present invention, the refining is LF refining. The present invention has no particular limitation on the operation of LF refining, which can be carried out according to means well known to those skilled in the art.

[0046] After refining, the obtained molten steel is subjected to a degassing treatment. The purpose of the degassing treatment is to control the removal of [H] and prevent hydrogen-induced cracking.

[0047] In the present invention, the degassing treatment may be VD degassing (vacuum degassing) or RH degassing (vacuum cycle degassing). The present invention has no particular limitation on the operation of the degassing treatment, and the degassing treatment may be performed according to methods well known to those skilled in the art.

[0048] After the degassing treatment is completed, according to the present invention, the obtained molten steel is cast to obtain a steel billet.

[0049] In the present invention, the casting speed, secondary cooling water ratio and electric stirring parameters will affect the performance of the final prepared steel. If the casting speed is too high or too low, the secondary cooling water ratio is too high or too low, and the electric stirring parameters are not within the satisfying range, it will have a certain impact on the macrostructure of the ingot (including the proportion of columnar crystals and equiaxed crystals), the segregation of the main component elements such as C, Mn, Cr, etc., and thus affect the comprehensive performance of the steel.

[0050] In summary, in the present invention, the casting is preferably carried out in a combined manner of crystallization electric stirring and terminal electric stirring; wherein the current intensity of the crystallization electric stirring is 100~200 A, preferably 120~150 A; the frequency is 1.0~1.5 Hz, preferably 1.2~1.3 Hz; the current intensity of the terminal electric stirring is 250~350 A, preferably 270~300 A; the frequency is 6.0~7.0 Hz, preferably 6.2~6.5 Hz.

[0051] At the same time, the present invention has been screened and the pouring speed is controlled to be 0.20~0.40 m / min, preferably 0.25~0.35 m / min; during the pouring process, the secondary cooling water volume is 0.12~0.18 L / Kg, preferably 0.13~0.16 L / Kg.

[0052] In addition, in the present invention, during the pouring process, the superheat of the molten steel is 18-30°C, preferably 20-25°C.

[0053] After the pouring is completed, the present invention performs rolling treatment on the obtained steel billet. Since the cooling speed of the billet is fast and the slow cooling effect is poor, the billet is prone to thermal stress and structural stress cracks. The present invention performs slow cooling treatment on the billet to ensure the slow cooling effect of the billet and avoid the occurrence of thermal stress and structural stress crack defects. In the present invention, the pit entry temperature of the slow cooling treatment is greater than 500°C, preferably greater than 520°C; the pit exit temperature is less than 150°C, preferably less than 130°C. The time of the slow cooling treatment is not less than 48h, preferably not less than 60h.

[0054] After the slow cooling treatment is completed, rolling is performed. The rolling is: heating the cast billet after pouring to obtain a hot cast billet, and rolling and cooling the hot cast billet.

[0055] In the present invention, the heating includes a preheating section, a first heating section, a second heating section and a soaking section.

[0056] Among them, the temperature of the preheating section is 500~1000℃, preferably 600~800℃, and the heating time is 2.5~3.0h; the temperature of the first heating section is 1050~1200℃, preferably 1100~1150℃, and the heating time is 1.5~2.0h; the temperature of the second heating section is 1200~1280℃, preferably 1220~1250℃, and the heating time is 1.5~2.0h; the temperature of the soaking section is 1190~1250℃, preferably 1200~1230℃, and the soaking time is 1.5~2.0h.

[0057] In the present invention, the total heating time is 7.0 to 9.0 h, preferably 7.5 to 8.5 h.

[0058] After the heating is completed, rolling is preferably performed. In the present invention, the starting rolling temperature of the rolling is 1110-1170°C, preferably 1120-1150°C; the final rolling temperature is 920-1020°C, preferably 950-1000°C. The rolling reduction ratio is ≥9, preferably ≥11.

[0059] After the rolling is completed, the steel can be obtained. According to the test, the annealed banded structure of the obtained steel is ≤2.0 level, and the grain size is ≥8.0 level, which can fully meet the standard requirements for the tooth shape accuracy and contact area accuracy of heavy truck axle gears after heat treatment.

[0060] In order to further illustrate the present invention, the following examples are provided for detailed description. The raw materials used in the following examples of the present invention are all common commercially available products.

[0061] Embodiment 1~3

[0062] This embodiment provides three types of heavy truck bridge gear steels, with specifications of φ500 mm ingot and φ150 mm steel. The specific composition design is shown in Table 1 below:

[0063] Table 1 Chemical composition of steel in each example (wt%)

[0064]

[0065] The preparation method is as follows:

[0066] It adopts converter, LF refining outside the furnace, RH vacuum degassing treatment process for smelting, continuous casting of round ingots, and rolling into finished products;

[0067] The pouring method is as follows: the pouring speed is controlled to be 0.36 m / min; the superheat of the molten steel is controlled to be 22℃~28℃; the secondary cooling water volume is controlled to be 0.16 L / Kg; the current intensity of the electric stirring of the crystallizer is controlled to be 150 A and the frequency is 1.5 Hz; the current intensity of the electric stirring at the end is controlled to be 300 A and the frequency is 7.0 Hz, and a φ500 mm round billet is obtained;

[0068] The rolling method is as follows:

[0069] The round billet is cold loaded into the heating furnace with a preheating section temperature of 860-920°C, a heating section temperature of 1070-1140°C, a second heating section temperature of 1240-1270°C, an even heating temperature of 1205-1235°C, a total heating time of 7.5h-8.5h, a start rolling temperature of 1130-1160°C, a final rolling temperature of 945-975°C, and a rolling compression ratio of ≥9.

[0070] After getting off the cooling bed, the steel is collected in time and put into the pit for slow cooling. The slow cooling temperature in the pit is greater than 500℃, the slow cooling time of the steel is 48h, and the temperature of the steel out of the temperature-controlled slow cooling pit is less than 150℃.

[0071] The specific process parameters of casting, rolling, and slow cooling of steel in Examples 1 to 3 are shown in Tables 2 to 4 below:

[0072] Table 2 Slab casting process control parameters

[0073]

[0074] Table 3 Rolling process control parameters

[0075]

[0076] Table 4 Control parameters of steel slow cooling process

[0077]

[0078] In the present invention, the isothermal annealed banded structure diagram of the steel obtained in Example 2 is as follows: Figure 1 As shown (100 μm); the grain size diagram of the steel obtained in Example 2 is as shown Figure 2 As shown (100 μm).

[0079] Example 4

[0080] Compared with Example 1, the difference is that the casting speed of the ingot is controlled to be 0.15 m / min, and the remaining parameters and steps refer to Example 1.

[0081] Example 5

[0082] Compared with Example 1, the difference is that the casting speed of the ingot is controlled to be 0.45 m / min, and the remaining parameters and steps refer to Example 1.

[0083] Comparative Example 1

[0084] Compared with Example 1, the difference is that the Al / N ratio is controlled to be 2.0, wherein the Al content is 0.025%, and the N content is 0.0125%. The remaining parameters and steps are the same as those in Example 1.

[0085] Comparative Example 2

[0086] Compared with Example 1, the difference is that the Al / N ratio is controlled to be 4.5, wherein the Al content is 0.042%, and the N content is 0.0093%. The remaining parameters and steps refer to Example 1.

[0087] The chemical compositions of the steels of Comparative Examples 1 and 2 are shown in Table 5:

[0088] Table 5 Chemical composition of steel of each comparative example (wt%)

[0089]

[0090] Performance Testing

[0091] The test method is as follows:

[0092] Hardenability test method: refer to GB / T 225-2006;

[0093] Grain size test method: refer to GB / T 6394-2017;

[0094] Banded tissue test method: refer to Q / CAM-10.1-201.

[0095] The test results are shown in Tables 6 and 7 below:

[0096] Table 6 Hardenability / HRC

[0097]

[0098] Table 7 Grain size, annealed banded structure / grade

[0099]

[0100] It can be seen from the data in Tables 6 and 7 that the steel for heavy truck axle gears prepared by the methods of Examples 1 to 5 of the present invention have greatly improved hardenability, grain size and annealed banded structure, which well meets the requirements of downstream customers for processing and manufacturing on hardenability, grain size and annealed banded structure.

[0101] Therefore, the steel for heavy truck axle gears of the present invention is designed and optimized for chemical composition, and the grains and structure are further refined by strictly controlling the Ca content in the steel, adding appropriate amounts of alloying elements Al and N, and strictly controlling the Al / N ratio, which can effectively improve the tooth shape accuracy and contact area accuracy of heavy truck axle gears after carburizing heat treatment. The contents not elaborated in detail in the present invention can all adopt the conventional technical knowledge in the field.

[0102] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A steel for heavy truck bridge gears, characterized in that: By mass fraction, it includes: 0.20~0.25% C, 0.17~0.35% Si, 0.75~0.95% Mn, 1.00~1.30% Cr, 0.025~0.045% Al, 0.35~0.45% Mo, 0.015~0.030% S, P≤0.012%, Ca≤0.0008%, Cu≤0.20%, O≤10×10 -4 %,90×10 -4 %≤N≤130×10 -4 % and H≤1.5×10 -4 %, the balance is Fe and impurities; Among them, 2.5≤Al / N≤4.

0.

2. The heavy truck bridge gear steel according to claim 1 is characterized in that: The heavy truck bridge gear steel includes, by mass fraction, 0.22-0.25% C, 0.17-0.30% Si, 0.80-0.95% Mn, 1.00-1.25% Cr, 0.025-0.040% Al, 0.35-0.40% Mo, 0.020-0.030% S, P≤0.011%, Ca≤0.0007%, Cu≤0.18%, O≤9×10 -4 %,85×10 -4 %≤N≤130×10 -4 % and H≤1.2×10 -4 %, the balance is Fe and impurities.

3. A method for preparing steel for heavy truck axle gears as claimed in claim 1 or 2, characterized in that: The following steps are involved: The steel for heavy truck axle gears is obtained by primary refining, LF refining, degassing treatment, casting and rolling.

4. The preparation method according to claim 3, characterized in that: The pouring speed is 0.20-0.40 m / min; During the pouring process, the superheat of the molten steel is 18-30°C; During the pouring process, the secondary cooling water volume is 0.12-0.18 L / Kg.

5. The preparation method according to claim 3 or 4, characterized in that: The pouring is carried out by combining crystallization electric stirring and terminal electric stirring; The current intensity of the crystallization electric stirring is 100-200 A, and the frequency is 1.0-1.5 Hz; The current intensity of the terminal electric stirring is 250~350 A, and the frequency is 6.0~7.0 Hz.

6. The preparation method according to any one of claims 3 to 5, characterized in that The rolling process includes heating the cast billet after pouring to obtain a hot cast billet, and rolling and cooling the hot cast billet.

7. The preparation method according to claim 6, characterized in that: The heating includes a preheating section, a first heating section, a second heating section and a soaking section; The temperature of the preheating section is 500-1000°C; The temperature of the heating stage is 1050-1200°C; The temperature of the second heating stage is 1200-1280°C; The temperature of the soaking section is 1190-1250°C; The total heating time is 7.0-9.0 h.

8. The preparation method according to claim 6 or 7, characterized in that: The starting rolling temperature of the rolling is 1110-1170°C, and the final rolling temperature is 920-1020°C; The rolling reduction ratio is ≥9.

9. The preparation method according to any one of claims 3 to 8, characterized in that Before the rolling, the cast ingot is subjected to a slow cooling treatment; The inlet temperature of the slow cooling treatment is greater than 500°C, and the outlet temperature is less than 150°C; The slow cooling treatment time is not less than 48 h.

10. The preparation method according to any one of claims 3 to 9, characterized in that: The primary refining is carried out by electric furnace primary refining or converter primary refining; The degassing treatment is VD degassing or RH degassing.

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