A high yield ratio carburized gear steel and its manufacturing method

Abstract

The present invention relates to the technical field of gear steel, and in particular to a high yield ratio carburized gear steel, wherein the components thereof include, by mass percentage: C: 0.15%-0.25%; Si≤0.35%; Mn: 0.60%-0.90%; P≤0.015%; S≤0.010%; Cr: 1.5%-1.8%; Ni: 1.5%-1.8%; Mo: 0.15%-0.35%; Nb: 0.02%-0.08%; A1≤0.035%; Cu: 0.025%-0.03%; N≤0.010%; O≤0.0015%, and the balance is Fe and impurity elements; the high yield ratio carburized gear steel has a tensile strength of ≥1200Mpa and a yield strength of ≥1130Mpa. By implementing the present scheme, a gear steel with a yield ratio greater than 0.94 is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of gear steel, and in particular to a high yield ratio carburized gear steel and a manufacturing method thereof. Background Art

[0002] Gear steel is a type of steel specially used for manufacturing gears and transmission components. Therefore, it needs to have a high yield strength and good toughness, so that it can withstand high loads during transmission and not easily break when impacted. This property is usually characterized by the yield ratio, that is, the yield ratio is the ratio of the yield strength of the material to the tensile strength. The yield ratio of gear steel is usually around 0.8, and the maximum does not exceed 0.9. If the yield ratio is higher than 0.9, it means that the yield strength of the material is close to or even exceeds its tensile strength. This will lead to a decrease in the material's plastic deformation ability, that is, a decrease in toughness, and the material is more likely to undergo brittle fracture when subjected to impact or dynamic loads. This is a serious disadvantage for transmission components such as gears that require high toughness. Precisely because of this shortcoming, there is little research at home and abroad on gear steel with a yield ratio higher than 0.9.

[0003] However, the applicant is currently developing a gear with special purposes, which requires a gear steel with a yield strength ratio greater than 0.94. However, current research in this field at home and abroad is almost blank, and there is no technical solution that can be referenced or borrowed. Therefore, it is urgent to study a gear steel with a yield strength ratio greater than 0.94. Summary of the invention

[0004] The present invention aims to provide a high yield ratio carburized gear steel and a manufacturing method thereof, so as to obtain a gear steel with a yield ratio greater than 0.94.

[0005] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a high yield ratio carburized gear steel, whose components, by mass percentage, include: C: 0.15%-0.25%; Si≤0.35%; Mn: 0.45%-0.90%; P≤0.015%; S≤0.010%; Cr: 1.5%-1.8%; Ni: 1.5%-1.8%; Mo: 0.15%-0.35%; Nb: 0.02%-0.08%; A1≤0.035%; Cu: 0.025%-0.03%; N≤0.010%; O≤0.0015%, and the balance is Fe and impurity elements; the high yield ratio carburized gear steel has a tensile strength ≥1200Mpa and a yield strength ≥1130Mpa.

[0006] A method for manufacturing high yield ratio carburized gear steel comprises the following manufacturing steps:

[0007] Step 1: Processing the component in claim 1 into an ingot;

[0008] Step 2: Forging the ingot at least three times, wherein the last forging temperature is 950°C-1150°C and the reduction is ≥15%;

[0009] Step 3: After forging, the workpiece is normalized twice, the first normalizing temperature is ≥960℃, and the second normalizing temperature is 910℃-950℃;

[0010] Step 4: After the steel has been subjected to the two normalizing treatments, it is machined and then subjected to carburizing heat treatment to obtain a high yield strength ratio carburized gear steel.

[0011] Preferably, as an improvement, in step 1, the components are smelted in a converter, then refined in a ladle, and finally refined by vacuum degassing.

[0012] Preferably, as an improvement, in step 2, the forging ratio of each forging is ≥1.5.

[0013] Preferably, as an improvement, in step 2, the total forging ratio is ≥6.

[0014] Preferably, as an improvement, in step 3, after the first normalizing is completed and air-cooled, the temperature is raised to 630°C-660°C and kept warm for a time calculated as 1.8 times the effective cross-sectional size of the forging, and then the temperature is raised to the second normalizing temperature for keeping warm for a time calculated as 1.5 times the effective cross-sectional size.

[0015] Preferably, as an improvement, in step 2, during the last forging, when the reduction amount increases, the forging temperature needs to be increased synchronously.

[0016] The principles and advantages of this solution are:

[0017] In order to develop carburized gear steel with a yield strength ratio greater than 0.94, after countless research experiments, we finally developed a gear steel with a high yield strength ratio by accurately configuring the components and the ratios between the components. The tensile strength of this gear steel is ≥1200Mpa, and the yield strength is ≥1130Mpa. Its yield strength ratio is much greater than 0.94. It has high stability under pressure, strong deformation resistance, and high structural safety.

[0018] Specifically, in order to make carburized gear steel with a yield ratio greater than 0.94, the C content is controlled at 0.15%-0.25%. Within this range, the hardenability of the gear steel is good. If it exceeds this range, the core ferrite increases, the bending fatigue performance of the material decreases, and it will not meet the toughness requirements of the gear core; at the same time, the Cr content is controlled between 1.50%-1.80% to further improve the hardenability of the gear steel and improve its mechanical properties. If it exceeds this range, it is not conducive to the control of carbides and will reduce the mechanical properties of the finished gear. Controlling the Ni content at 1.5%-1.8% can effectively avoid brittle fracture of the material at low temperature and low load. If it is lower than this range, the low-temperature impact performance of the gear steel will be greatly reduced. The Mo content is controlled at 0.15%-0.35%. On the one hand, it can cooperate with Ni to further improve the low-temperature impact performance of the gear steel, and at the same time improve its hardenability and wear resistance. The Al content is controlled below 0.035%, which helps to better form AlN with the N element, thereby further refining the grains and improving the strength and toughness of the steel. However, if the Al content in the steel is greater than 0.035%, it is easy to increase the chance of inclusions in the steel and reduce the impact performance of the material. If the Nb content is controlled between 0.02% and 0.08%, it can form a finer precipitate phase, thereby inhibiting the recrystallization of the steel, thereby effectively refining the grains. However, if the Nb content in the steel exceeds this range, coarse NbC particles will be formed during the high-temperature tempering process, which will reduce the impact toughness of the gear steel. If the N content is controlled below 0.01%, it can combine with microalloys such as Al and Nb in the steel to form MN-type precipitates, which can pin the grain boundaries at high temperatures and inhibit the growth of austenite grains. However, when the N element content in the steel is greater than 0.01%, it will precipitate and enrich in the steel prematurely, reducing the effect of inhibiting grain growth and reducing the toughness of the steel. The O element content is controlled below 0.0015% to reduce the number and size of inclusions in the steel, thereby significantly improving the fatigue life of the gear steel.

[0019] At the same time, this solution controls the contents of Nb, Al, and N elements to generate as many niobium carbonitride and aluminum nitride inclusions as possible in the steel, avoiding the generation of niobium carbide (NbC), while increasing the Al content in the steel, reducing the dissolved oxygen in the steel, reducing the generation of non-metallic inclusions, reducing metallurgical defects, and improving the mechanical properties of the material.

[0020] In summary, the present invention strictly controls the content of each component to ensure that the tensile strength of the gear steel is ≥1200Mpa, the yield strength is ≥1130Mpa, and the yield strength ratio is much greater than 0.94.

[0021] In addition, the present invention also provides a preparation method for the high yield ratio carburized gear steel, by strictly controlling the final forging temperature and the amount of pressure, wherein the final forging temperature is controlled at 950°C-1150°C, the amount of pressure reduction is ≥15%, and the changes in the two are positively correlated, that is, when the amount of pressure reduction increases, the temperature needs to increase synchronously, otherwise the internal damage will be uneven. At the same time, the amount of pressure reduction in this scheme cannot be less than 15% to prevent the increase of grains, so that the internal structure is more evenly broken and the grain size is better refined. At the same time, the present invention adopts two normalizing, so that the ingot with uniform internal crushing after forging is combined with two normalizing, which can further even the structure and composition, refine the grains, and better control the precipitation of AlN (aluminum nitride) to improve the yield strength of the gear steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a distribution diagram of aluminum nitride inclusion precipitation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0023] The following is further described in detail through specific implementation methods:

[0024] Program Overview:

[0025] A high yield ratio carburized gear steel. In this embodiment, gear steel grades 17CrNiMo6 and 18CrNiMo7-6 are taken as examples. The components thereof include, by mass percentage: C: 0.15%-0.25%; Si≤0.35%; Mn: 0.45%-0.90%; P≤0.015%; S≤0.010%; Cr: 1.5%-1.8%; Ni: 1.5%-1.8%; Mo: 0.15%-0.35%; Nb: 0.02%-0.08%; A1≤0.035%; Cu: 0.025%-0.03%; N≤0.010% (100ppm); O≤0.0015% (15ppm), and the remainder is Fe and impurity elements; the high yield ratio carburized gear steel has a tensile strength ≥1200Mpa and a yield strength ≥1130Mpa.

[0026] The mass percentage of each component includes but is not limited to the following values:

[0027] C: 0.15%, 0.17%, 0.19%, 0.20%, 0.22%, 0.23%, 0.25%, etc.;

[0028] Si: 0.35%, 0.33%, 0.31%, 0.3%, 0.28%, 0.24%, 0.2%, 0.17%, 0.12%, etc.

[0029] Mn: 0.45%, 0.7%, 0.8%, 0.90%, etc.;

[0030] P: 0.015%, 0.013%, 0.011%, 0.01%, 0.007%, 0.002%, etc.

[0031] S: 0.01%, 0.006%, 0.003%, 0.001%, etc.

[0032] Cr: 1.5%, 1.6%, 1.7%, 1.8%, etc.;

[0033] Ni: 1.5%, 1.6%, 1.7%, 1.8%, etc.;

[0034] Mo: 0.15%, 0.18%, 0.20%, 0.22%, 0.25%, 0.28%, 0.3%, 0.33%, 0.35%, etc.;

[0035] Nb: 0.02%, 0.04%, 0.06%, 0.08%;

[0036] A1: 0.035%, 0.03%, 0.028%, 0.025%, 0.02%, etc.

[0037] Cu: 0.025%, 0.027%, 0.028%, 0.03%;

[0038] N: 0.010%, 0.009%, 0.008%, 0.007%, etc.

[0039] O: 0.0015%, 0.0013%, 0.0011%, 0.001%, etc.

[0040] A method for manufacturing high yield ratio carburized gear steel comprises the following manufacturing steps:

[0041] Step 1: The high yield ratio carburized gear steel is smelted in a converter, then refined in a ladle, and finally refined into ingots by vacuum degassing. In this scheme, ladle refining and vacuum degassing are used to improve the purity of the molten steel. The purer the molten steel, the fewer internal defects of the material and the better the mechanical properties.

[0042] Step 2: Forge the ingot three times. The first two forgings are performed in the conventional way. The forging temperature of the last forging needs to be controlled between 950℃-1150℃, and the reduction (deformation) of the ingot is ≥15%. When the reduction increases, the final forging temperature also needs to increase accordingly.

[0043] Step 3: After forging, the workpiece is normalized twice, wherein the first normalizing temperature is ≥960°C, and the second normalizing temperature is 910°C-950°C. Specifically, after the first normalizing is completed and air-cooled, the temperature is raised to 630°C-660°C for insulation, and the insulation time is calculated as 1.8 times the effective cross-sectional size of the forging. The temperature is then raised to the second normalizing temperature for insulation, and the insulation time is calculated as 1.5 times the effective cross-sectional size.

[0044] Step 4: After the steel has been subjected to the two normalizing treatments, it is machined and then subjected to carburizing heat treatment to obtain a high yield strength ratio carburized gear steel.

[0045] This scheme uses a 7-ton steel ingot for testing, as follows:

[0046] 1. The specific preparation method of the present invention is used to process gear steel with different components, wherein Examples 1 to 3 are gear steels processed by the component manufacturing method of the present invention; while the components of Comparative Examples 1 to 8 are different from those of the present invention. The specific components of each embodiment and comparative example are shown in Table 1 (unit: O, N are ppm, and the others are wt%):

[0047]

[0048]

[0049] Table 1

[0050] 2. The specific components of the present invention are used below to process gear steel by different preparation methods, wherein Examples 4 to 6 are gear steels processed by the manufacturing method of the present invention; and the preparation methods of Comparative Examples 9 to 12 are different from those of the present invention, as shown in Table 2:

[0051] Final forging temperature(℃) Final forging reduction (%) Normalizing times Example 4 950 15％ 2 Example 5 1050 18％ 2 Example 6 1150 20％ 2 Comparative Example 10 950 15％ 1 Comparative Example 11 930 15％ 2 Comparative Example 12 950 10％ 2 Comparative Example 13 1200 15％ 2

[0052] Table 2

[0053] The test data of the gear steel prepared in each embodiment and comparative example are as follows:

[0054]

[0055]

[0056] Table 3

[0057] It can be seen from Table 3 that by precisely controlling the components and the ratios between the components, the tensile strength of the gear steel can be ≥1200Mpa, the yield strength ≥1130Mpa, and the yield strength ratio can reach 0.95, or even as high as 0.98. In Comparative Examples 10-13, the yield strength ratios are all lower than 0.88, and in Comparative Example 11, when the terminal temperature is too low, cracks will appear on the surface of the gear steel. In Comparative Example 12, when the reduction amount is too low, the grain size is coarse level five, and in Comparative Example 13, when the terminal temperature is too high, the grain size is coarse level four. Due to the poor grain refinement of the two comparative examples, the yield strength ratio of the gear steel cannot meet expectations. Through the preparation process of the present method, the internal structure of the steel can be broken more evenly during the preparation process, the grain size can be better refined, and at the same time, the precipitation of AlN (aluminum nitride) can be better controlled, such as Figure 1 As shown, it is the distribution diagram of aluminum nitride inclusion precipitation in this scheme. It can be seen from the figure that the aluminum nitride is evenly dispersed, which has the effect of refining the grains and strengthening the dispersion of the gear steel, and effectively improves the yield strength of the gear steel.

[0058] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A high yield ratio carburized gear steel, characterized in that: Its components by mass percentage include: C: 0.15%-0.25%; Si≤0.35%; Mn: 0.45%-0.90%; P≤0.015%; S≤0.010%; Cr: 1.5%-1.8%; Ni: 1.5%-1.8%; Mo: 0.15%-0.35%; Nb: 0.02%-0.08%; A1≤0.035%; Cu: 0.025%-0.03%; N≤0.010%; O≤0.0015%, the remainder being Fe and impurity elements; the tensile strength of the high yield ratio carburized gear steel is ≥1200Mpa, and the yield strength is ≥1130Mpa.

2. A method for manufacturing high yield ratio carburized gear steel, characterized in that: The manufacturing steps include: Step 1: Processing the component in claim 1 into an ingot; Step 2: Forging the ingot at least three times, wherein the last forging temperature is 950°C-1150°C and the reduction is ≥15%; Step 3: After forging, the workpiece is normalized twice, the first normalizing temperature is ≥960℃, and the second normalizing temperature is 910℃-950℃; Step 4: After the steel has been subjected to the two normalizing treatments, it is machined and then subjected to carburizing heat treatment to obtain a high yield strength ratio carburized gear steel.

3. The method for manufacturing a high yield ratio carburized gear steel according to claim 2, characterized in that: In step 1, the components are smelted in a converter, then refined in a ladle, and finally refined by vacuum degassing.

4. The high yield ratio carburized gear steel and the manufacturing method thereof according to claim 3, characterized in that: In step 2, the forging ratio of each forging is ≥ 1.

5.

5. The high yield ratio carburized gear steel and the manufacturing method thereof according to claim 4, characterized in that: In step 2, the total forging ratio is ≥6.

6. The high yield ratio carburized gear steel and the manufacturing method thereof according to claim 5, characterized in that: In step 3, after the first normalizing is completed and air-cooled, the temperature is raised to 630°C-660°C for insulation, and the insulation time is calculated as 1.8 times the effective cross-sectional size of the forging. The temperature is then raised to the second normalizing temperature for insulation, and the insulation time is calculated as 1.5 times the effective cross-sectional size.

7. The high yield ratio carburized gear steel and the manufacturing method thereof according to claim 6, characterized in that: In step 2, during the last forging, when the reduction amount increases, the forging temperature needs to be increased simultaneously.

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