JP2024048190A - Nitriding steel with excellent machinability and nitriding properties - Google Patents

Abstract

[Problem] To provide a nitriding steel having a good balance between workability, hardness after nitriding, and core hardness. [Solution] The steel is composed of, in mass%, C: 0.12-0.30%, Si: 0.10-0.40%, Mn: 0.50-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50-2.00%, Mo: 0.03-0.30%, Al: 0.025-0.300%, N: 0.004-0.030%, V: 0.05-0.30%, the balance being Fe and unavoidable impurities, and has a composition of 0.15<0.10×Cr+0.67×Al+0.2 A nitriding steel in which 4 x V < 0.43, 0.25 < 0.80 x C + 0.10 x Cr + 0.36 x Mo < 0.50, the pre-nitriding structure is a structure consisting of ferrite and pearlite, or a structure containing bainite in addition to ferrite and pearlite, the ratio of pearlite in these structures is 10 to 60% by area and the ratio of bainite is 0 to 20% by area, and the core hardness is 160 to 260 Hv. [Selected drawing] None

Description

The present invention relates to a nitriding steel that is suitable as a material for machine structural parts, such as gears for power transmission parts in automobiles, construction machinery, machine tools, etc., which are used after hot forging and then surface hardening treatment, such as gas nitriding or gas soft nitriding, in which nitrogen penetrates into the steel surface.

Nitriding steels that aim to achieve both strength and toughness have been proposed as machine structural steels used for power transmission parts such as gears.
For example, the applicant of the present application has invented a steel having superior surface hardness (800 Hv or more) and hardened layer depth (0.25 mm or more) compared to conventional nitriding steels by optimizing the steel composition (by mass%, C: 0.10-0.40%, Si: 0.10-0.50%, Mn: 0.50-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50-1.50%, Mo: 0.05-0.30%, Al: 0.15-0.80%, N: 0.005-0.030%, balance Fe), converting the pre-nitriding structure to martensite by quenching and tempering, and specifying the type and ratio of nitrides to be N _Al /N _Cr of 0.2 or more (see Patent Document 1 and Patent No. 6,300,647).

In addition, the steel composition (C: 0.20-0.30%, Si: 0.25% or less, Mn: less than 0.50%, P: 0.03% or less, S: 0.05% or less, Cr: over 1.00% to 2.00%, Mo: 0.10% to less than 0.50%, V: 0.10% to 0.50%, Al: over 0.10% to 0.20%, Ti: 0.10% or less, N: 0.0060% to 0.020% or less, and 18<2 By specifying the conditions of 7C+9Mn+7Cr+8Mo and 22<37C+6Mn+8Cr+11V, with the remainder being Fe, and specifying the hot forging and heat treatment conditions, a bainite-type nitrided part has been proposed that has a bainite structure of 70% or more, a surface hardness after nitriding of 650Hv or more, and a hardening depth of 0.2mm or more, defined as a range of 450Hv or more (see Patent Document 2).

In addition, to ensure surface hardness and effective hardened layer depth, a nitriding steel has been proposed that contains the following steel components: C: 0.05-0.30%, Si: 0.003-0.50%, Mn: 0.4-3.0%, Cr: 0.2-0.9%, Al: 0.19-0.70%, V: 0.05-1.0%, and Mo: 0.05-0.50%, satisfying 0.5%≦1.9Al+Cr≦1.8%, with the balance being Fe (see Patent Document 3).

JP 2015-229780 A JP 2006-022350 A JP 2010-147224 A

The above-mentioned conventional proposals have had problems such as high amounts of Al, V, Ti, etc. added, insufficient hardness of the core structure, and strict regulations on the thickness of the compound layer.
For example, the nitriding steel described in Patent Document 1 has a martensite structure before nitriding and contains high amounts of C, Al and V, so that from the standpoint of machinability and processability, the production of parts is costly.

In addition, since the nitriding steel described in Patent Document 2 has a large Cr content, it is necessary to increase the V content and the nitriding temperature in order to increase the hardening depth, which raises concerns about an increase in raw material costs and insufficient surface hardness.In addition, the core hardness becomes excessive, which raises concerns about deterioration of machinability, etc.

In addition, the nitriding steel described in Patent Document 3 has a reduced amount of Cr, which has a negative effect on hardening depth, and large amounts of V, Al, and Mo must be added to compensate for the reduced core hardness, which increases raw material costs.

In this way, Al and Cr, which can form nitrides and increase the surface hardness through nitriding, will reduce the hardening depth if added in large quantities. In addition, martensitic transformation cannot be utilized in the nitriding process, which requires heat treatment at a temperature below the _A1 line. Therefore, in order to ensure the core hardness of power transmission parts and the like, it is necessary to add further alloying elements in a creative way, but adding excessive alloying elements is likely to impair processability such as machinability.

Therefore, taking into consideration the background of nitriding steel for mechanical structures described above, the present invention aims to provide a nitriding steel that has a good balance between workability, hardness after nitriding, and core hardness.

As a result of extensive research, the inventors of the present application have optimized the balance of Al, V, and Cr components to obtain a nitriding steel that has excellent properties after nitriding without impairing workability. After hot forging, the alloy is designed to have a mixed phase structure of ferrite and pearlite or a mixed phase structure of ferrite and pearlite with some bainite, providing an excellent balance between machinability and nitridability and allowing alloy nitrides to precipitate appropriately during nitriding. A structure with a higher bainite ratio provides excellent core hardness but poor machinability. However, the present invention provides a mixed phase structure of ferrite and pearlite or a mixed phase structure of ferrite and pearlite with some bainite, resulting in better machinability.

Therefore, the first means for solving the problems of the present invention is to
In mass %, it consists of C: 0.12 to 0.30%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 2.00%, Mo: 0.03 to 0.30%, Al: 0.025 to 0.300%, N: 0.004 to 0.030%, V: 0.05 to 0.30%, the balance being Fe and unavoidable impurities,
The steel for nitriding satisfies the relationship between Formula 1 and Formula 2, and has a pre-nitriding structure consisting of ferrite and pearlite, or a structure containing bainite in addition to ferrite and pearlite, in which the ratio of pearlite in these structures is 10 to 60% in terms of area ratio and the ratio of bainite is 0 to 20% in terms of area ratio, and the core hardness is 160 to 260 Hv.
0.15<0.10×Cr+0.67×Al+0.24×V<0.43...Equation 1
0.25<0.80×C+0.10×Cr+0.36×Mo<0.50...Equation 2
However, the element symbols in Formulas 1 and 2 are substituted with the mass % values of the corresponding components.

The second method is:
In mass%, C: 0.12 to 0.30%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 2.00%, Mo: 0.03 to 0.30%, Al: 0.025 to 0.300%, N: 0.004 to 0.030%, V: 0.05 to 0.30%,
Furthermore, the steel contains one or more of the following optional additional components: Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less;
The balance is Fe and unavoidable impurities.
The steel for nitriding satisfies the relationship between Formula 1 and Formula 2, and has a pre-nitriding structure consisting of ferrite and pearlite, or a structure containing bainite in addition to ferrite and pearlite, in which the ratio of pearlite in these structures is 10 to 60% in terms of area ratio and the ratio of bainite is 0 to 20% in terms of area ratio, and the core hardness is 160 to 260 Hv.
0.15<0.10×Cr+0.67×Al+0.24×V<0.43...Equation 1
0.25<0.80×C+0.10×Cr+0.36×Mo<0.50...Equation 2
However, the element symbols in Formulas 1 and 2 are substituted with the mass % values of the corresponding components.

According to the present invention, a nitriding steel having a good balance between workability, hardness after nitriding, and core hardness can be obtained. That is, the nitriding steel of the present invention has a core hardness of 160 to 260 Hv and is excellent in workability.
Furthermore, when the nitriding steel of the present invention is nitrided, the surface hardness after nitriding is 700 Hv or more, and the hardened layer depth of the surface layer having a hardness of 400 Hv or more is 0.29 mm or more, so that the steel can also have a hardness after nitriding.

Before explaining the embodiments of the present invention, the reasons for specifying the chemical composition of the steel for nitriding will be described. Note that the percentages of the chemical composition are mass percentages.

C: 0.12 to 0.30%
C is a component that increases the hardness of the material. If C is less than 0.12%, the core hardness after nitriding is insufficient and decreases, resulting in insufficient strength as a part. On the other hand, if C is too much, the material hardness increases and workability decreases. In other words, machinability and cold workability become poor. It also inhibits the diffusion of nitrogen from the steel surface, reducing the depth of the hardened layer. Therefore, C is set to 0.30% or less.

Si: 0.10 to 0.40%
Silicon is a useful component for deoxidation during steel manufacturing. If the silicon content is less than 0.10%, deoxidation during manufacturing is likely to be insufficient, resulting in a decrease in the inclusion quality. On the other hand, if the silicon content exceeds 0.40%, the material hardness increases, resulting in a decrease in workability. Therefore, the silicon content is set to 0.10 to 0.40%.

Mn: 0.50 to 1.50%
Since Mn is an ingredient that improves the core hardness (hardness of the material), if the Mn content falls below 0.50%, the core hardness will be insufficient. On the other hand, if Mn increases the material hardness, it will hinder workability such as machinability and forgeability, resulting in a decrease in workability. Therefore, the Mn content is set to 0.50 to 1.50%.

Cr: 0.50 to 2.00%
Cr is an ingredient that improves the core hardness of steel and also improves the hardness when the steel is nitrided. If there is too little Cr, the hardness after nitriding will be insufficient, and the core hardness will also be insufficient. On the other hand, if there is too much Cr, the material hardness will increase, resulting in a decrease in workability. In addition, the diffusion of nitrogen will be hindered, resulting in a decrease in the depth of the hardened layer.
Therefore, the Cr content is set to 0.50 to 2.00%, and preferably, the Cr content is set to 1.00 to 1.80%.

Mo: 0.03 to 0.30%
Mo is a component that improves the hardness of the material. If the Mo content is less than 0.03%, the core hardness after nitriding is reduced, resulting in insufficient strength. On the other hand, if the Mo content exceeds 0.30%, the workability is reduced due to the increase in the material hardness, resulting in poor machinability and cold workability.

Al: 0.025 to 0.300%
Al is a useful component for deoxidation during manufacturing. If there is too little Al, deoxidation during steel manufacturing is likely to be insufficient, and the inclusion quality will decrease. In addition, the surface hardness and hardened layer depth of the steel parts after nitriding will also be insufficient. If there is too much Al, coarse nitrides (AlN) will form, which will decrease fatigue properties and workability. Therefore, Al is set to 0.025 to 0.300%. Desirably, Al is 0.050 to 0.200%.

N: 0.004 to 0.030%
N is a component that forms carbonitrides. If there is too little N, there will be a shortage of carbonitrides and the crystal grains will become coarse, which will lead to a decrease in toughness and fatigue properties. On the other hand, if there is too much N, coarse carbonitrides will be formed, which will result in a decrease in fatigue properties and workability, and the pinning effect will not be exerted due to a decrease in fine carbonitrides that have a pinning effect, leading to coarsening of the crystal grains. Therefore, N is set to 0.004 to 0.030%.

V: 0.05 to 0.30%
V is a useful component for improving the hardness of steel, and since it has little effect in inhibiting nitrogen diffusion, it is also a useful component for preventing a decrease in the depth of the hardened layer. Al and V act in combination, V forms clusters with N to ensure the diffusion of N, and nitrides of Al and N are generated there, which increases the hardened layer depth, so the hardened layer depth is greatly improved. If there is too little V, the hardened layer depth will be insufficient. On the other hand, if there is too much V, the workability will deteriorate and the cost will increase due to the V component. Therefore, V is set to 0.05 to 0.30%.

The balance is Fe and unavoidable impurities. Note that since both P and S may be contained as unavoidable impurities, the upper limits are specified as follows:

P: 0.030% or less (unavoidable impurities)
P is a component that promotes grain boundary segregation and reduces toughness, so when P is contained as an unavoidable impurity, the P content is set to 0.030% or less.

S: 0.030% or less (unavoidable impurities)
If there is an excessive amount of S, a large amount of coarse MnS is formed, which leads to a decrease in toughness and fatigue strength. Therefore, if S is contained, the S content is set to 0.030% or less.

The optional additional components in the present invention are explained below. One or more of Nb, Ti, and B can be selected and added to the components of the steel of the present invention within the following ranges.

Nb: 0.10% or less Nb is a component that improves hardness, but if it is excessive, the workability deteriorates as the hardness increases. Therefore, when Nb is contained, the content is set to 0.10% or less.

Ti: 0.020 to 0.200%
Ti is a component that improves hardness by precipitating nitrides and carbonitrides. If Ti is less than 0.020%, the amount of fine nitrides is insufficient, and Ti does not sufficiently improve bending fatigue strength. On the other hand, if Ti is excessive, the amount of coarse carbonitrides increases, and bending fatigue strength decreases. Therefore, when Ti is added, the amount is set to 0.020 to 0.200%.

B: 0.0030% or less B is a component that increases the hardness of the material, but if it exceeds 0.0030%, the workability decreases as the hardness of the material increases. In addition, embrittlement due to the formation of carboborides is also caused. Therefore, when B is added, it is set to 0.0030% or less.

Next, the reasons for defining formulas 1 and 2 will be described.
0.15<0.10×Cr+0.67×Al+0.24×V<0.43...Equation 1
0.25<0.80×C+0.10×Cr+0.36×Mo<0.50...Equation 2
The formula is calculated by substituting the mass percentage of the corresponding steel component for the element symbol.

First, formula 1 is an index using Cr, Al, and V, and is an index related to nitriding hardness and hardened layer depth. If the value of formula 1 is 0.15 or less, the nitriding hardness and hardened layer depth will be insufficient. If it is 0.43 or more, the hardened layer depth will also be likely to be insufficient.

Formula 2 is an index using C, Cr, and Mo, and is an index related to the core hardness and material hardness after nitriding. If the value of Formula 2 is 0.25 or less, the core hardness will be insufficient, resulting in insufficient strength. If the value of Formula 2 is 0.50 or more, the material hardness will be too high, resulting in poor workability of the part (machinability and cold workability).

The structure before nitriding is a structure consisting of ferrite and pearlite, or a structure consisting of ferrite and pearlite further containing bainite, and the proportion of pearlite is 10 to 60% in terms of area percentage, and the proportion of bainite is 0 to 25%.The steel for nitriding of the present invention has a structure before nitriding treatment which is a ferrite + pearlite structure, or a structure consisting of ferrite + pearlite containing bainite, and the proportion of pearlite in the structure is 10 to 60% in terms of area percentage, and the proportion of bainite is 0 to 20% in terms of area percentage.
If the proportion of pearlite is less than 10%, the structure will be mainly composed of soft ferrite, making it difficult to cut chips and reducing machinability, whereas if the proportion of pearlite is more than 60%, the hardness will be excessive and the machinability will be reduced.
On the other hand, if the proportion of bainite exceeds 20%, the core hardness becomes too high, resulting in poor workability. Therefore, the proportion of bainite, if present, is set to 20% or less in terms of area ratio.

Core hardness: 160-260Hv
If the material hardness is too hard, the workability will be poor. On the other hand, if the core hardness after nitriding is insufficient, the strength of the entire part such as a gear will be insufficient. Therefore, the preferred core hardness is 160 to 260 Hv. A hardness in this range is preferable because it does not deteriorate the machinability and does not require any special effort on the tool side.

100 kg of steels with the chemical compositions shown in Examples 1 to 19 and Comparative Examples 1 to 5 in Table 1, with the balance being Fe and unavoidable impurities, were melted in a vacuum melting furnace. Next, steel bars with a diameter of 40 mm were produced by hot forging, and then normalized by holding at a temperature of 700 to 950°C for 1 to 3 hours, followed by air cooling. In the examples, the cooling rate was 0.1 to 0.3°C/s.

Normalizing is performed using a Kanthal furnace, specifically following the procedure below. First, each test material with the above composition is placed in a furnace set to a designated holding temperature, and the test materials are allowed to heat up for 30 minutes. They are then held for a desired period of time before being air-cooled or water-cooled. The holding time is selected taking into consideration the amount and dimensions of the steel material to be charged into the furnace.

After normalizing, the material was then subjected to nitriding treatment at 580°C for 2 to 6 hours.

Next, the area ratio (%) of pearlite in the steel structure, the area ratio (%) of bainite, the surface hardness after nitriding, the hardened layer depth after nitriding, and the core hardness (hardness after normalizing) were measured using the following procedure, and are shown in Table 2.

<Evaluation method>
The hardness distribution from the surface of each steel in Table 1 was measured using a Vickers hardness tester in accordance with JIS Z 2244, and the surface hardness and effective hardened layer depth of each example steel and each comparative steel shown in Table 1 are shown in Table 2.
(1) Surface Hardness The surface hardness was defined as the hardness at a depth of 0.05 mm from the surface, and was measured using a Vickers hardness tester in accordance with JIS Z 2244.
(2) Depth of Surface Hardened Layer The hardness distribution from the surface was measured using a Vickers hardness tester in accordance with JIS Z 2244, and the area maintaining a hardness of 400 Hv or more was taken as the depth of the hardened layer.
(3) Core Hardness after Nitriding The test piece after nitriding was cut, and the core hardness of the cross section was measured by a Vickers hardness tester in accordance with JIS Z 2244.

In addition, the structure of each steel before nitriding was identified and classified by observing it under an optical microscope after mirror polishing of each normalized steel and etching with nital solution. The ratio of each structure was calculated by counting the number of pixels in the captured microscope images using image processing software, and the area ratio of pearlite and bainite in the total structure are shown in Table 2 as a percentage.

The steels of Examples 1 to 19 of this application have excellent workability, with an area ratio of pearlite structure of 10 to 60%, a bainite structure of 20% or less, and a core hardness of 160 to 260 Hv. Furthermore, when these steels are nitrided, the surface hardness after nitriding is 700 Hv or more, and a practical nitrided layer is formed with a hardened layer depth of 0.29 mm or more, making these nitriding steels well-balanced with workability. Therefore, by subjecting parts machined with these steels to nitriding treatment, machine structural parts, particularly power transmission parts such as gears, can be obtained.

Comparative Example No. 1 has excessive C and Mn, exceeds the value of formula 2, and has an excessive bainite structure. The core hardness is too high, resulting in poor workability. The hardened layer depth is also insufficient.
In Comparative Example No. 2, the amounts of Al and V were too small, and the depth of the hardened layer was insufficient.
In Comparative Example No. 3, the values of both Formula 1 and Formula 2 are below the required values, and the test is not satisfactory, with low surface hardness and insufficient core hardness.
Comparative Example No. 4 has an insufficient amount of Mo, an excessive pearlite ratio, an insufficient core hardness, and an insufficient hardened layer depth after nitriding.
In Comparative Example No. 5, the value of Formula 1 is lower than that, and the surface hardness and hardened layer depth after nitriding are insufficient.

Claims (2)

In mass %, it consists of C: 0.12 to 0.30%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 2.00%, Mo: 0.03 to 0.30%, Al: 0.025 to 0.300%, N: 0.004 to 0.030%, V: 0.05 to 0.30%, the balance being Fe and unavoidable impurities,
A steel for nitriding which satisfies the relationship between Formula 1 and Formula 2, and has a pre-nitriding structure consisting of ferrite and pearlite, or a structure containing bainite in addition to ferrite and pearlite, in which the ratio of pearlite in these structures is 10 to 60% in terms of area ratio and the ratio of bainite is 0 to 20% in terms of area ratio, and the core hardness is 160 to 260 Hv.
0.15<0.10×Cr+0.67×Al+0.24×V<0.43...Equation 1
0.25<0.80×C+0.10×Cr+0.36×Mo<0.50...Equation 2
However, the element symbols in Formulas 1 and 2 are substituted with the mass % values of the corresponding components. In mass%, C: 0.12 to 0.30%, Si: 0.10 to 0.40%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 2.00%, Mo: 0.03 to 0.30%, Al: 0.025 to 0.300%, N: 0.004 to 0.030%, V: 0.05 to 0.30%,
Furthermore, the steel contains one or more of the following optional additional components: Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less;
The balance is Fe and unavoidable impurities.
A steel for nitriding which satisfies the relationship between Formula 1 and Formula 2, and has a pre-nitriding structure consisting of ferrite and pearlite, or a structure containing bainite in addition to ferrite and pearlite, in which the ratio of pearlite in these structures is 10 to 60% in terms of area ratio and the ratio of bainite is 0 to 20% in terms of area ratio, and the core hardness is 160 to 260 Hv.
0.15<0.10×Cr+0.67×Al+0.24×V<0.43...Equation 1
0.25<0.80×C+0.10×Cr+0.36×Mo<0.50...Equation 2
However, the element symbols in Formulas 1 and 2 are substituted with the mass % values of the corresponding components.