JP2010265487A - Martensitic stainless steel and rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a martensitic stainless steel having ≥HV1400 surface hardness and ≥HV650 inner part hardness. <P>SOLUTION: This martensitic stainless steel contains, by weight, 0.35-0.45% C, ≤0.2% Si, ≤0.2% Mn, ≤0.01% P, ≤0.01% S, 15.5-16.5% Cr, 1.5-2.5% Mo, 0.001-0.0015% B, 0.15-0.25% N and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a martensitic stainless steel for rolling bearings having high strength excellent in corrosion resistance and wear resistance.

  Conventionally, martensitic stainless steels such as SUS440A, SUS440B, and SUS440C are used for bearings and blades because of their high hardness. Incidentally, in rolling bearings that similarly require high hardness, nitriding treatment may be performed in order to increase wear resistance. This type of rolling bearing stainless steel is known from US Pat.

  Further, after martensitic stainless steel such as SUS440A, SUS440B, and SUS440C is quenched and tempered, a nitriding treatment may be applied to further increase the surface hardness and improve wear resistance. . At this time, since the hardness of the surface layer is about HV900, when silicon nitride is used for the rolling elements (balls) of the rolling bearing, wear of the outer ring and the inner ring may be increased. In order to avoid this, if the outer ring and the inner ring are also made of silicon nitride, it becomes very expensive and difficult to produce.

  Nitriding techniques include plasma nitriding and gas nitriding. When gas soft nitriding or plasma nitriding is performed, it is performed at a temperature around 600 ° C. However, if it is processed at this temperature, the temperature becomes considerably higher than the tempering temperature, so that the internal hardness decreases to about HV300. If softened to this extent, the members of the inner and outer rings of the rolling bearing may be crushed by high contact stress received from the rolling elements.

  Therefore, in order to perform nitriding at a lower temperature, a method of performing nitriding after performing fluorination (for example, NV nitriding by Air Water) is known from Patent Document 2. In addition, it is known in Patent Documents 3 to 5 that the stainless steel for rolling bearings in which the rolling nitriding member is subjected to NV nitriding treatment has a surface hardness of HV900 or more and an internal hardness of HV600 or more.

Japanese Patent No. 3326912 Japanese Patent No. 34287776 (JP-A-8-193256) JP 2001-74053 A JP 2001-187916 A WO98 / 44270 Publication

  According to Patent Documents 3 to 5, stainless steel having a surface hardness of HV900 or more and an internal hardness of HV600 or more is obtained. However, when silicon nitride balls are used for the rolling elements of the rolling bearing, the surface hardness from the outermost surface to a depth of 10 μm is HV1400 or more at present so that it can withstand higher contact surface pressure. There is a demand for martensitic stainless steel having a hardness of up to 40 μm and HV1200, and further having an internal hardness of HV650 or more.

  Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a martensitic stainless steel and a rolling bearing having higher surface hardness and appropriate internal hardness.

  The inventor diligently developed a martensitic stainless steel capable of achieving the above object. As a result, the Si content is 0.2% or less and the Mn content is 0.2% with respect to a specific range of C. The following ranges, P is 0.01% or less, S is 0.01% or less, Cr is 15.5 to 16.5%, Mo is 1.5 to 2.5%. By adding quenching, sub-zero treatment, and high temperature tempering treatment at around 500 ° C. to the martensitic stainless steel each containing in the range and the balance being Fe and inevitable impurities, the internal hardness increases, and By performing the NV nitriding treatment, the surface hardness from the surface to a depth of 40 μm is as high as HV1200 or more, and there is little dimensional change due to the amount of retained austenite and aging. It found that it is possible to obtain a martensitic stainless steel.

  The martensitic stainless steel of the present invention has been made based on the above findings, and the components are in a weight ratio, C in the range of 0.35 to 0.45%, Si in the range of 0.2% or less, Mn is 0.2% or less, P is 0.01% or less, S is 0.01% or less, Cr is 15.5 to 16.5%, Mo is 1.5 to 2 It is characterized by containing 0.5%, B in the range of 0.001 to 0.0015% and N in the range of 0.15 to 0.25%, the balance being Fe and inevitable impurities.

  According to the martensitic stainless steel of the present invention, N is an important element that precipitates CrN during tempering and prevents a decrease in hardness due to tempering. Moreover, by containing B in the range of 0.001 to 0.0015%, it is effective for improving the strength, and the hardenability is enhanced, and the effect of improving the strength and toughness due to the precipitation of BN can be obtained further. Martensitic stainless steel can be provided.

  In the invention described in claim 2 of the present invention, the strength and toughness are further improved by adding W in the range of 0.18 to 0.22% by weight as a component in the invention of claim 1. The martensitic stainless steel made can be provided.

  The invention described in claim 3 is characterized in that the stainless steel of the present invention is activated with fluorine gas and then subjected to nitriding treatment at 400 to 600 ° C., preferably 450 to 550 ° C.

  The invention described in claim 4 is a martensitic stainless steel having a surface hardness of HV1400 or more and an internal hardness of HV650 or more after the nitriding treatment. The hardness indicates the target value of the present invention.

  Further, in the invention described in claim 5, by using the martensitic stainless steel of the invention of claim 1, 2, 3 or 4 as a bearing material, surface hardness can be obtained by quenching, sub-zero treatment, and tempering treatment. It is possible to provide a rolling bearing suitable for use in a corrosive atmosphere that is capable of HV650 or more, is superior in corrosion resistance, cold workability and wear resistance to SUS440C, has little residual austenite amount and dimensional change due to aging. it can.

  Next, the basis of the content of chemical components (elements) in the present invention will be described. In addition, the following% is a weight ratio. In the present invention, the remainder of the components listed below is considered to be Fe and impurities inevitably mixed.

・ C (carbon): 0.35-0.45%
C is an austenite-forming element, and if added in a large amount, eutectic carbide tends to be formed and cracking is likely to occur, so that sufficient corrosion resistance cannot be obtained, so the upper limit was made 0.45%. In order to obtain sufficient quenching hardness, the lower limit of C is set to 0.35%.

-Si (Silicon): 0.2% or less Si containing 0.2% or less, if containing Si in an amount exceeding 0.2%, the toughness is remarkably deteriorated and harmful to hot workability. It was decided to make it contain in the range.

Mn (manganese): 0.2% or less Mn is an austenite stabilizing element, and excessive addition increases the amount of retained austenite, not only lowering the surface hardness after heat treatment, but also harming corrosion resistance. It tends to cause dimensional changes over time. Therefore, the Mn content is set to a range of 0.2% or less.

P (phosphorus): 0.01% or less P is a component that precipitates at grain boundaries and causes cold brittleness, and the content is made 0.01% or less in order to avoid cold brittleness.

-S (sulfur): 0.01% or less Since S deteriorates corrosion resistance or impairs hot workability, the content is made 0.01% or less.

-Cr (chromium): 15.5 to 16.5%
Cr, like Mn, has the effect of increasing the solid solubility limit of N and improving the corrosion resistance. This is when 13% or more is added. In the present invention, since C is defined as low as 0.35 to 0.45%, the lower limit is set to 15.5% so that Cr is not increased and the corrosion resistance is not lowered. In order to form a strong passive film and to secure an austenite single phase at the quenching temperature, the upper limit was made 16.5%. Incidentally, when the content exceeds 16.5%, Cr, and generates the M ₂₃ C ₆ type carbide which is a carbide of Fe, resulting in or deteriorate the corrosion resistance or to reduce the hardness after heat treatment.

Mo (molybdenum): 1.5 to 2.5%
Mo, like Cr, has the effect of increasing the solid solubility limit of N, improving the corrosion resistance and improving the hardenability, but in order to obtain such an effect, addition of 1.5% or more is necessary. It is. Moreover, since excessive addition causes the fall of toughness, the upper limit was made into 2.5%.

・ N (nitrogen): 0.15 to 0.25%
N is an element that is very effective in improving the surface hardness and corrosion resistance after heat treatment of martensitic stainless steel. To obtain this effect, N is 0.15% or more, preferably 0.18%. Need more. In addition, in the case of atmospheric dissolution, there is no blow (bubbles) in the material, and the solid solution limit is 0.25% for a martensitic stainless steel that can be used practically, so the upper limit is 0.25%. It was.

B (boron): 0.001% to 0.0015%
When B is added, BN precipitates, which is effective for improving the strength and enhances the hardenability. However, in order to obtain this effect, addition of 0.001% or more is necessary. On the other hand, excessive addition causes a decrease in toughness, so the upper limit of the addition amount is made 0.0015%.

  The above are the essential components of the alloy of the present invention. In addition to these, the alloy of the present invention may contain W.

W (tungsten): 0.18% to 0.22%
Since W has a large atomic radius, W acts as a solid solution strengthening element. In order to obtain this effect, addition of 0.18% or more is necessary. Moreover, since excessive addition causes the fall of toughness, the upper limit of addition amount shall be 0.22%.

  The martensitic stainless steel of the present invention can provide martensitic stainless steel having a surface hardness of HV1400 or more and an internal hardness of HV650 or more. In addition to being excellent in corrosion resistance and cold workability, the amount of retained austenite is small, dimensional change due to aging is small, and cracking does not occur in quenching and subsequent subzero treatment. In addition, it is extremely promising as a material for rolling bearings that require a high level of wear resistance and corrosion resistance.

It is a tempering curve after the subzero process of an Example, Comprising: It is a diagram which shows the relationship between surface hardness and tempering temperature. It is a diagram which shows the hardness distribution after the nitriding process of an Example. It is the (a) front view and (b) side view which show typically the rolling fatigue testing machine used for the rolling fatigue test done in the Example.

Hereinafter, the present invention will be described by way of examples.
Using a 10 kg high-frequency induction furnace, each alloy of the chemical components (% by weight) shown in Table 1 is melted, and after homogeneous heating, the solidified ingot is cut, and a sample of a φ20 round bar is obtained by hot forging. Produced. Examples 1 to 3 are materials obtained by adding B and N, and Comparative Example 1 is a material obtained by removing B from the components of Examples 1 to 3. Further, Comparative Examples 2 and 3 are SUS440C and SUS420J2 which are JIS standard steel types, respectively. In addition to these chemical components shown in Table 1, these samples all contain Fe and impurities that are inevitably mixed.

  These samples were kept at 1075 ° C. for 1 hour and then water-cooled, followed by sub-zero treatment that was put into liquid nitrogen.

(1) Tempering temperature and surface hardness FIG. 1 shows the tempering curves after quenching and sub-zero treatment of Examples 1 to 3 and Comparative Examples 1 to 3. In FIG. 1, the horizontal axis represents the tempering temperature (° C.), and the vertical axis represents the surface hardness (HV). Here, no NV nitriding treatment is applied to any of the examples and comparative examples. In order to investigate the influence of the tempering temperature on the surface hardness, the tempering temperature was changed from 150 ° C. to 800 ° C., and a sample for each tempering temperature was obtained. The surface hardness was measured using a Vickers hardness tester according to “Vickers hardness test-test method” defined in JIS-Z2244.

  According to FIG. 1, in Examples 1 to 3 and Comparative Example 1 in which about 0.2% of N is added, the surface hardness decreases when the tempering temperature is up to about 300 ° C., but beyond that, by secondary curing. The surface hardness increases and takes a maximum value around 500 ° C. This is presumably due to the precipitation of CrN. Moreover, Examples 1-3 have shown that surface hardness becomes so high that the addition amount of B is large. This is presumably due to the precipitation of BN.

  Particularly in Comparative Example 1, the initial surface hardness is low despite the addition of 0.2% of N. It is considered that sufficient hardenability was not obtained because B was not added. That is, when B is not contained, a sufficient cooling rate cannot be obtained, and a portion that has not undergone complete martensitic transformation appears, so that the surface hardness is low. Although the comparative example 2 does not contain N, initial hardness is high, but this is because the content of C is high. Moreover, the comparative examples 2 and 3 which do not contain N do not show secondary hardening, and surface hardness will be HV300 or less in the vicinity of 500 degreeC.

(2) Hardness after nitriding treatment Next, for Examples 1 to 3 and Comparative Examples 1 to 3, after quenching, sub-celling treatment, and tempering, oxygen in the surface layer was substituted with fluorine, and then N was diffused The NV nitriding treatment (trade name of Air Water Co., Ltd.) was performed at 500 ° C. The result of measuring the hardness from the sample surface to the inside in this case is shown in FIG.

  According to FIG. 2, the surface hardness of both the example and the comparative example decreases from the surface toward the inside, and the hardness becomes a substantially constant value at a depth of about 80 μm or more. In the present invention, this almost constant hardness is regarded as internal hardness. In Examples 1 to 3 and Comparative Example 1, since N is added, the internal hardness is HV650 or more due to precipitation of CrN. Moreover, in Examples 1-3, the surface hardness has shown the high value of HV1400 or more. Furthermore, the surface hardness from the surface to a depth of 40 μm is a high value of HV1200 or higher.

  In order to make the surface hardness HV1400 or more and the surface hardness up to a depth of 40 μm also HV1200 or more, N exceeding the solid solubility limit of N is added to distort the crystal lattice more greatly. Although it becomes possible, in normal atmospheric dissolution, blow occurs when N of 0.25% or more is added. Therefore, if special equipment such as a pressurized ESR furnace is used, a large amount of N can be added. However, this greatly increases the cost of the material. However, in the present invention, by adding a small amount of B, BN precipitates, the surface hardness is HV1400 or more without adding N 0.25% or more, and the surface hardness up to a depth of 40 μm is HV1200 or more, A material with higher wear resistance can be obtained.

(3) Rolling fatigue test Eight cylindrical test pieces of φ12 mm × L22 mm were produced from the materials of Examples 1 to 3 and Comparative Examples 1 to 3, respectively. These test pieces were held at 1050 ° C. for 1 hour and then cooled with water, followed by NV nitriding at 500 ° C., and then subjected to a rolling fatigue test. The rolling fatigue test was performed using a rolling fatigue tester shown in FIG. 3 under the conditions of a contact surface pressure of 6 GPa and a rotational speed of 23130 rpm. As the opponent ball, a steel ball having a diameter of 3/4 inch (19.05 mm) was used. Turbine oil # 68 was used as the lubricating oil.

The rolling fatigue test results were plotted on Weibull distribution probability paper, L ₁₀ life showing ₁₀ % failure probability and L ₅₀ life showing 50% failure probability were determined, and rolling fatigue life was evaluated. as a result

  According to Table 2, Examples 1 to 3 show a higher rolling fatigue life than Comparative Examples 1 to 3. Comparative Example 1 has the highest lifetime value among the comparative examples, but shows a lower lifetime value than any of the Examples. This is probably because B was not added as compared with the examples, and thus quenching was insufficient and the martensite was not partially transformed. Further, Comparative Examples 2 and 3 show lower lifetime values than Examples 1 to 3 and Comparative Example 1, but since neither N nor B is added, the surface and internal hardness are low. it is conceivable that.

Claims (5)

  The components are, by weight ratio, C in the range of 0.35 to 0.45%, Si in the range of 0.2% or less, Mn in the range of 0.2% or less, P in the range of 0.01% or less, S in a range of 0.01% or less, Cr in a range of 15.5 to 16.5%, Mo in a range of 1.5 to 2.5%, B in a range of 0.001 to 0.0015%, N In a range of 0.15 to 0.25%, the balance being made of Fe and inevitable impurities, martensitic stainless steel.   The martensitic stainless steel according to claim 1, further comprising W in a range of 0.18 to 0.22% by weight as a component.   3. The martensitic stainless steel according to claim 1, wherein the martensitic stainless steel is subjected to nitriding treatment at 400 to 600 ° C. after being activated with fluorine gas.   The martensitic stainless steel according to claim 3, wherein the surface hardness after nitriding is HV1400 or more and the internal hardness is HV650 or more.   A rolling bearing characterized in that a material constituting at least one of an inner ring, an outer ring and a rolling element is made of the martensitic stainless steel according to any one of claims 1 to 4.

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