Nitrogen-containing martensitic stainless bearing steel and method for manufacturing the same

Abstract

The present application relates to a kind of nitrogen-containing martensitic stainless steel and its preparation method, wherein, a high-nitrogen martensitic stainless bearing steel, characterized in that, according to mass percentage, including the following components: C: 0.60~0.75%, Cr: 16.0~18.0%, Mo: 0.50~0.65%, N: 0.15~0.25%, Si: ≤0.80%, Mn: ≤0.50%, S: ≤0.30%, P: ≤0.35%, the balance is Fe and inevitable impurities.The nitrogen-containing martensitic stainless bearing steel in the application adds 0.15~0.35% of nitrogen element, promotes the dispersion distribution of eutectic precipitate, effectively reduces the precipitation of primary carbide, and the C of nitrogen-containing martensitic stainless bearing steel interacts with Cr and Mo and other elements, and significantly improves the strength and toughness of the material, solves the problem of poor toughness of high-carbon chromium martensitic stainless bearing steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgy, and particularly relates to a nitrogen-containing martensitic stainless bearing steel and a preparation method thereof. BACKGROUND

[0002] High-carbon chromium stainless bearing steel is the most widely used stainless bearing steel, and its typical steel grades are 9Cr18 and 9Cr18Mo. Compared with 9Cr18, 9Cr18Mo has higher hardness and tempering resistance. Both 9Cr18 and 9Cr18Mo are high-carbon high-chromium martensitic stainless steels, which can achieve high hardness and wear resistance after quenching. However, because of the high carbon content and chromium content, there are inevitably large eutectic precipitates and network or belt-shaped eutectic precipitates (carbides) along the grain boundaries, which greatly reduce the plasticity, toughness and corrosion resistance of the bearing steel, so that the toughness of this kind of bearing steel is very poor, and the unnotched impact toughness is only about several dozen joules, and the U-shaped notch toughness is only a few joules.

[0003] For example, Chinese invention patent "High-hardness high-wear-resistance high-nitrogen martensitic stainless bearing steel and preparation method thereof", patent number ZL201610712698.9 (authorized publication number CN106086631B) discloses a high-hardness high-wear-resistance high-nitrogen martensitic stainless bearing steel with the following chemical composition by weight: C: 0.65% to 1.25%, Cr: 13.00% to 20.00%, Mo: 0.15 to 4.50%, N: 0.05 to 0.50%, V: 0.03 to 1.20%, Nb≤0.1%, Si≤1.00%, Mn≤1.00%, the rest being Fe and unavoidable impurities, and Ti≤0.0020%, Al:≤0.008%, P≤0.010%, S≤0.008%, Cu≤0.25%, Ni≤0.30%, Ca≤0.001%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and 0.8%≤C+N≤1.50%. The high-nitrogen martensitic stainless bearing steel in the above patent has high hardness, but the martensitic stainless bearing steel in the above patent contains a certain amount of expensive metal elements such as V and Nb. Generally, Nb and V are added in a small amount as micro-alloying elements to achieve the effect of grain refinement and precipitation strengthening. The maximum addition amount of V in the above patent reaches 1.20%, which on the one hand significantly increases the cost of the alloy, and on the other hand, excessive addition of V elements easily forms a large amount of vanadium carbide and vanadium nitride precipitates, which brings difficulties to the subsequent heat treatment and processing of the steel, and together with Nb, easily forms Fe-Nb-C eutectic and Fe-Nb eutectic, reducing the plasticity and toughness of the steel.

[0004] For example, the Chinese invention patent application "High-molybdenum martensitic stainless steel for cutters with better toughness and corrosion resistance than 9Cr18MoV and method", with patent application number CN201911264220.4 (application publication number CN110923579A), discloses a high-molybdenum martensitic stainless steel, which comprises an alloy structure composed of the following chemical components and their weight percentage contents: carbon C = 0.65-0.85, chromium Cr = 14-16, molybdenum Mo = 2.00-3.50, manganese Mn = 0.8-1.5, vanadium V = 0.05-0.50, niobium Nb = 0.05-0.15, silicon Si = 0.10-0.30, sulfur S ≤ 0.010, phosphorus P ≤ 0.015, and the balance being iron Fe. Although the toughness and corrosion resistance of the martensitic stainless steel are improved in the above patent, the steel contains a large amount of Mo and Mn elements. Too high Mo will increase the tendency of decarburization of the steel, and too high Mn will reduce the oxidation resistance of the steel, increase the overheating sensitivity and temper brittleness of the steel.

[0005] For example, the Chinese invention patent "Corrosion method for displaying original austenite grain boundaries of high-nitrogen martensitic stainless steel", with patent number ZL201710285036.2 (authorization publication number CN107014661B), discloses a high-nitrogen martensitic stainless steel with a composition range of: C: 0.2-0.6%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 15.0-18.0%, Mo: 0.5-1.5%, N: 0.25-0.5%, and Fe: balance. The C content in the above patent is between 0.2-0.6%, and the total amount of C% + N% is between 0.45-1.1%. Since the solubility of nitrogen in martensite is very low, the nitrogen content in the above patent is relatively high. In order to obtain the yield of nitrogen, i.e. to increase the solubility of nitrogen in the martensitic bearing steel, special smelting means such as pressurized induction melting or pressurized electroslag remelting need to be used, which will inevitably increase the difficulty of smelting and manufacturing cost.

[0006] Therefore, there is a need to further improve the existing martensitic stainless bearing steel and its preparation method. SUMMARY

[0007] The first technical problem to be solved by the present application is to provide a nitrogen-containing stainless bearing steel with high strength, high hardness and high impact toughness in view of the current situation of the prior art.

[0008] The second technical problem to be solved by the present application is to provide a preparation method of the above high-nitrogen martensitic stainless bearing steel.

[0009] The technical scheme adopted by the present application to solve the first technical problem is: a high-nitrogen martensitic stainless bearing steel, characterized by comprising the following components in percentage by mass: C: 0.60-0.75%, Cr: 16.0-18.0%, Mo: 0.50-0.65%, N: 0.15-0.25%, Si: ≤0.80%, Mn: ≤0.50%, S: ≤0.30%, P: ≤0.35%, and the balance being Fe and inevitable impurities.

[0010] C: Carbon is one of the main elements to ensure the hardness, strength and wear resistance of the bearing steel. The hardness of the martensite is mainly determined by the carbon content. The higher the content of the carbon element dissolved in the matrix, the greater the hardness of the martensite. However, the increase of the carbon content is easy to cause segregation of the steel and thus produce liquid carbon, banded carbide and network carbide in the structure. These uneven carbides have an adverse effect on the fatigue life of the bearing steel, especially the formation of large block eutectic carbides by liquidation of the carbides, which greatly damages the fatigue life of the bearing steel. In addition, carbon in the stainless bearing steel forms carbide with chromium. The higher the carbon content, the more chromium carbide is formed, the lower the chromium content in the solid solution, and the lower the electrode potential of the steel, and thus the lower the corrosion resistance. Therefore, the interaction of C, Cr and N elements is fully considered, the content of C is controlled between 0.60-0.75%, and the content of C+N is controlled between 0.75-1.0%.

[0011] Cr: Chromium is a ferrite-forming element. When the chromium content is large enough, the steel will become a single ferrite stainless steel. The chromium content in the stainless bearing steel cannot be too high, otherwise stable austenite phase region cannot be obtained, and thus martensite cannot be obtained by quenching. In addition, one of the most important roles of chromium in stainless bearing steel is to improve the corrosion resistance of the steel. When the chromium content of the steel is relatively low, the corrosion depth decreases with the increase of the chromium content. When the chromium content exceeds 12%, the corrosion depth decreases to zero. Therefore, the chromium content of various stainless steels, including stainless bearing steel, is above 12%. The present application fully considers the influence of Cr element on phase structure, corrosion resistance, hardness and interaction with C, and controls the Cr content in the range of 16.0-18.0%, so as to guarantee the hardenability, high hardness and high corrosion resistance of the steel.

[0012] Mo: Molybdenum, like chromium, is also a ferrite-forming element, and their ferrite-forming abilities are comparable. In the stainless bearing steel, molybdenum can improve the strength and hardness of the steel, and can also form M2C type carbide with carbon in the steel, which is precipitated at a higher temperature to produce secondary hardening of the steel and improve the high-temperature performance of the steel. In addition, Mo can also improve the pitting resistance of the steel, so the content of Mo is controlled between 0.50-0.65%.

[0013] N: The carbon and chromium content of the martensitic stainless steel is high, which causes the formation of large eutectic carbides in the structure, thereby reducing the fatigue life of the material. When nitrogen replaces part of the carbon, the hardness and strength of the material will not decrease, but due to the reduction of carbon content and the corresponding reduction of chromium content, large eutectic carbides do not appear in the structure, and the nitride or carbonitride precipitated phase is also fine and dispersed, so the fatigue life of the material can be greatly improved. In addition, the M 23 C6 formed by traditional martensitic stainless steel such as 9Cr18 consumes a large amount of chromium, and the carbonitride or nitride formed after the addition of nitrogen consumes less chromium content, so nitrogen is beneficial to improving the corrosion resistance of the steel. The present application fully considers the role of nitrogen element in the stainless martensitic bearing steel and its solubility, which not only ensures that fine and dispersed carbonitride or nitride can be obtained, but also avoids the occurrence of subcutaneous bubbles during the solidification process of the steel. The addition amount of nitrogen must be fully considered in coordination with other elements and processes to ensure that nitrogen exists in a solid solution state or in the form of fine and dispersed carbonitride or nitride. The content of nitrogen in the present application is controlled at 0.15-0.25%.

[0014] Si: Silicon is a ferrite forming element in stainless steel, which dissolves in ferrite to improve the hardness and strength of the steel, but the increase of silicon content will reduce the plasticity of the steel. The content of silicon in the present application is controlled within 0.5%.

[0015] Mn: Manganese is a weak austenite forming element, but it is a strong austenite structure stabilizing element in stainless steel and can improve the solubility of nitrogen in steel.

[0016] The room temperature yield strength of the nitrogen-containing martensitic stainless bearing steel is 1100-1300 MPa, the room temperature tensile strength is 1600-1720 MPa, the elongation is 10.0-15.0%, the hardness HRC is 55-62, and the impact energy KU2 is 45-55 J. In this way, the nitrogen-containing martensitic stainless bearing steel has high strength, high hardness and high impact toughness.

[0017] The technical scheme adopted by the present application to solve the second technical problem is: a preparation method of the high-nitrogen martensitic stainless bearing steel, characterized by comprising the following steps in sequence:

[0018] 1) The raw materials are weighed according to the above proportions and then melted and cast to form an ingot;

[0019] 2) The ingot is subjected to forging treatment to form a rod;

[0020] 3) The rod of step 2) is subjected to heat treatment.

[0021] Preferably, the ingot of step 1) is kept at a temperature T1 = 1000-1200℃ for a holding time t1 of 3-5h before the forging treatment in step 2) is performed.

[0022] Preferably, the open-die temperature in the forging treatment is T1, and the finish-die temperature is 940-960℃, which is slightly higher than the finish-die temperature of high-carbon martensitic bearing steel such as 9Cr18Mo. The finish-die temperature is increased to increase the forging ratio, which is beneficial to uniform structure and grain refinement, and to increase the yield.

[0023] Preferably, the temperature T1 is 1100℃, the holding time t1 is 4h, and the finish-die temperature is 950℃.

[0024] Preferably, in step 3), the bar is annealed at a temperature T2 of 850-880℃ for a holding time t2 of 2-4h, heated to a temperature T3 of 1100-1150℃ for a holding time t3 of 1.5-2.5h, oil-cooled to room temperature, and then low-temperature tempered at a temperature T4 of 170-190℃ for a holding time t4 of 1.5-2.5h.

[0025] Further preferably, the temperature T2 is 870℃, the holding time t2 is 3h, the temperature T3 is 1130℃, the holding time t3 is 2h, the temperature T4 is 180℃, and the holding time t4 is 2h.

[0026] Compared with the prior art, the nitrogen-containing martensitic stainless bearing steel of the present application has the following advantages: by adding 0.15-0.25% of nitrogen element, the martensitic stainless bearing steel can be prepared by conventional smelting or smelting under nitrogen protection, and considering the role and solubility of nitrogen element in the martensitic stainless bearing steel, the steel can obtain fine and dispersed carbonitride or nitride, avoid the occurrence of subcutaneous bubbles during solidification, promote the dispersion of eutectic precipitates, effectively reduce the precipitation of primary carbide, and the interaction and cooperation of C, Cr and Mo elements in the nitrogen-containing martensitic stainless bearing steel significantly improve the strength and toughness of the material, solve the problem of poor toughness of high-carbon chromium martensitic stainless bearing steel, and the impact toughness is increased by about 10 times compared with 9Cr18 and 9Cr18Mo. In addition, the bearing steel can be widely used in the demand for martensitic stainless bearing steel in high-end fields such as aerospace, weapons and ships, and high-end precision lathes. DETAILED DESCRIPTION

[0027] The present application will be further described in detail below with reference to the examples.

[0028] Example 1

[0029] The nitrogen-containing martensitic stainless bearing steel of the embodiment contains, in mass percentage, C: 0.60%, Cr: 17.5%, Mo: 0.65%, N: 0.20%, Si: 0.35%, Mn: 0.33%, and the balance of Fe and inevitable impurities, wherein C+N=0.80%.

[0030] The preparation method of the nitrogen-containing martensitic stainless bearing steel comprises the following steps in sequence:

[0031] 1) The raw materials are weighed according to the above proportions and then melted and cast to form an ingot;

[0032] 2) The ingot in step 1) is forged into a φ30mm rod at a temperature T1=1100℃ and a holding time t1 of 4h, with an open forging temperature of 1100℃ and a final forging temperature of 950℃;

[0033] 3) The rod in step 2) is heat treated: first, the rod is annealed at a temperature T2 of 870℃ and a holding time t2 of 3h, then heated to a temperature T3 of 1130℃ and held for a time t3 of 2h, and then oil-cooled to room temperature, followed by low-temperature tempering at a temperature T4 of 180℃ and a holding time t4 of 2h.

[0034] The nitrogen-containing martensitic stainless bearing steel prepared in the embodiment has the following room temperature properties: a tensile strength Rm of 1750MPa, a strength value Rp0.2 corresponding to a plastic elongation of 0.2% of 1280MPa, an elongation A of 13%, a hardness HRC of 60, and an impact energy KU2 of 48J. 0.2

[0035] Embodiment 2:

[0036] The difference between the embodiment and the above-mentioned embodiment 1 is only that the component content of the nitrogen-containing martensitic stainless bearing steel is different, and specifically, the nitrogen-containing martensitic stainless bearing steel of the embodiment contains, in mass percentage, C: 0.65%, Cr: 17.5%, Mo: 0.65%, N: 0.25%, Si: 0.32%, Mn: 0.23%, and the balance of Fe and inevitable impurities, wherein C+N=0.90%.

[0037] The nitrogen-containing martensitic stainless bearing steel prepared in the embodiment has the following room temperature properties: a tensile strength Rm of 1650MPa, a strength value Rp0.2 corresponding to a plastic elongation of 0.2% of 1180MPa, an elongation A of 14%, a hardness HRC of 58, and an impact energy KU2 of 52J. 0.2

[0038] Embodiment 3:

[0039] ​​The difference between the present embodiment and the above-mentioned embodiment 1 is only that the component content of the nitrogen-containing martensitic stainless bearing steel is different, and specifically, the nitrogen-containing martensitic stainless bearing steel of the present embodiment contains, by mass percent, C: 0.70%, Cr: 16.5%, Mo: 0.62%, N: 0.15%, Si: 0.30%, Mn: 0.25%, and the balance of Fe and inevitable impurities, wherein C+N=0.85%.

[0040] The room temperature performance of the nitrogen-containing martensitic stainless bearing steel prepared in the present embodiment is as follows: the tensile strength Rm is 1680 MPa, the strength value Rp0.2 corresponding to the plastic elongation of 0.2% is 1200 MPa, the elongation A is 13.5%, the hardness HRC is 61, and the impact energy KU2 is 50 J. 0.2 The room temperature performance of the nitrogen-containing martensitic stainless bearing steel prepared in the present embodiment is as follows: the tensile strength Rm is 1680 MPa, the strength value Rp0.2 corresponding to the plastic elongation of 0.2% is 1200 MPa, the elongation A is 13.5%, the hardness HRC is 61, and the impact energy KU2 is 50 J.

[0041] Embodiment 4:

[0042] The difference between the present embodiment and the above-mentioned embodiment 1 is only that the component content of the nitrogen-containing martensitic stainless bearing steel is different, and specifically, the nitrogen-containing martensitic stainless bearing steel of the present embodiment contains, by mass percent, C: 0.75%, Cr: 16%, Mo: 0.50%, N: 0.25%, Si: 0.50%, Mn: 0.40%, S: 0.15%, P: 0.3%, and the balance of Fe and inevitable impurities, wherein C+N=1.0%.

[0043] The process parameters used in the preparation method of the nitrogen-containing martensitic stainless bearing steel are different, and specifically, the temperature T1 is 1000°C, the holding time t1 is 5h, the temperature T2 is 850°C, the holding time t2 is 2h, the temperature T3 is 1100°C, t3 is 1.5h, the temperature T4 is 190°C, and the holding time t4 is 2.5h.

[0044] Embodiment 5:

[0045] The difference between the present embodiment and the above-mentioned embodiment 1 is only that the component content of the nitrogen-containing martensitic stainless bearing steel is different, and specifically, the nitrogen-containing martensitic stainless bearing steel of the present embodiment contains, by mass percent, C: 0.75%, Cr: 18%, Mo: 0.50%, N: 0.25%, Si: 0.80%, Mn: 0.50%, S: 0.30%, P: 0.35%, and the balance of Fe and inevitable impurities, wherein C+N=1.0%.

[0046] The process parameters used in the preparation method of the nitrogen-containing martensitic stainless bearing steel are different, specifically, the temperature T1 is 1200℃, the holding time t1 is 3h, the temperature T2 is 880℃, the holding time t2 is 4h, the temperature T3 is 1150℃, t3 is 2.5h, the temperature T4 is 170℃, and the holding time t4 is 1.5h.

[0047] Example 6

[0048] The difference between the present example and the above-mentioned example 1 is only that the component content of the nitrogen-containing martensitic stainless bearing steel is different, and specifically, the nitrogen-containing martensitic stainless bearing steel of the present example contains, in terms of mass percentage, C: 0.61%, Cr: 18%, Mo: 0.50%, N: 0.24%, Si: 0.80%, Mn: 0.50%, S: 0.30%, P: 0.35%, and the balance of Fe and inevitable impurities, wherein C+N=0.9%.

[0049] Comparative Example

[0050] 9Cr18Mo is used, and the chemical composition thereof contains, in terms of mass percentage, C: 0.95-1.10%, Cr: 16.0-18.0%, Mo: 0.40-0.70%, Si: ≤0.80%, and Mn: ≤0.80%. The same preparation method as that of example 1 is used, and the room temperature performance of the prepared 9Cr18Mo is as follows: the tensile strength Rm is 1780MPa, the strength value Rp 0.2 0.2% corresponding to the plastic elongation is 1330MPa, the elongation A is 1.5%, the hardness HRC is 58, and the impact energy KU2 is 3.0J.

[0051] Compared with the above-mentioned comparative example, it can be seen that the elongation and impact energy of each example of the present application are both increased by more than 10 times on the premise that the tensile strength, Rp 0.2 0.2% and the hardness HRC are basically equivalent.

Claims

1. A method for preparing high-nitrogen martensitic stainless bearing steel, characterized in that, High-nitrogen martensitic stainless bearing steel comprises the following components by mass percentage: C: The high-nitrogen martensitic stainless bearing steel has the following composition: 0.60–0.75% Cr: 16.0–18.0% Mo: 0.50–0.65% N: 0.15–0.25% Si: ≤0.80% Mn: <0.50% S: ≤0.30% P: ≤0.35%, with the balance being Fe and unavoidable impurities. The room temperature yield strength is 1100–1300 MPa, the room temperature tensile strength is 1600 MPa–1720 MPa, the elongation is 10.0–15.0%, the hardness is HRC 55–62, and the impact energy (KU2) is 45–55 J. The preparation method includes the following steps: 1) Weigh the raw materials according to the above proportions and then melt and cast them to form ingots; 2) The ingot is forged into a bar stock; 3) Heat treat the bar stock from step 2): Anneal the bar stock at a temperature T2 of 850-880℃ for a holding time t2 of 2-4h. After annealing, heat the bar stock to a temperature T3 of 1100-1150℃ for a holding time t3 of 1.5-2.5h, then oil cool to room temperature, and subsequently perform low-temperature tempering at a temperature T4 of 170-190℃ for a holding time t4 of 1.5-2.5h.

2. The preparation method according to claim 1, characterized in that: Before forging in step 2), the ingot from step 1) is held at a temperature of 1000-1200℃ for 3-5 hours.

3. The preparation method according to claim 1, characterized in that: The initial forging temperature during forging is T1, and the final forging temperature is 940–960℃.

4. The preparation method according to claim 3, characterized in that: The temperature T1 is 1100℃, the holding time t1 is 4h, and the final forging temperature is 950℃.

5. The preparation method according to claim 1, characterized in that: In step 3), the bar is annealed at a temperature T2 of 850-880℃ for a holding time t2 of 2-4h. After annealing, the bar is heated to a temperature T3 of 1100-1150℃ for a holding time t3 of 1.5-2.5h, then oil-cooled to room temperature, and subsequently tempered at a temperature T4 of 170-190℃ for a holding time t4 of 1.5-2.5h.

6. The preparation method according to claim 5, characterized in that: Temperature T2 is 870℃, holding time t2 is 3h, temperature T3 is 1130℃, holding time t3 is 2h, temperature T4 is 180℃, holding time t4 is 2h.