JP2024013079A - Carbonitriding steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbonitriding steel having high strength and superior toughness after carbonitriding treatment.

SOLUTION: A carbonitriding steel is adopted that includes, in mass%, C: 0.10-0.30%, Si: 0.50% or less, Mn: 1.15-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, Cr: 0.20-0.64%, N: 0.001-0.020%, Mo: 0.32-0.60%, and O: 0.0030% or less, and satisfies the following formula (1), with the Mn content in cementite being 1.70 mass% or more. The formula (1) is: 0.12×Mn-0.62×Cr+3.15×Mo≥0.80 mass% (1) where, Mn, Cr and Mo each denote the content of the element in mass%.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to steel materials for carbonitriding, and more specifically to steel materials to be used after carbonitriding treatment.

Machine structural parts such as automobile parts, industrial machine parts, and construction machine parts are sometimes subjected to surface hardening treatment to improve fatigue strength, for example.

Among various surface hardening treatments, carburizing treatment is often applied to mechanical structural parts such as those described above. A hardened layer is formed on the surface of the carburized component. This hardened layer not only provides high fatigue strength, but also provides high toughness because the core of the component is also heat treated.

In recent years, instead of performing only carburizing treatment, carbonitriding treatment, in which nitriding is performed after carburizing treatment, is sometimes performed. A hardened layer is formed by carburizing, and nitrogen is introduced into the hardened layer by nitriding, thereby increasing the softening resistance. Thereby, it is possible to suppress softening of the hardened layer when heat is generated during use of the component, and high fatigue strength can be maintained even during use.

On the other hand, by introducing nitrogen into the hardened layer, there is a concern that toughness may decrease due to the formation of nitrides. This is because steel commonly used for carburizing treatment contains Si, Cr, and Ti, and these elements tend to form nitrides by combining with nitrogen. In particular, when the steel contains Ti, V, Nb, or B, the nitrides become coarse and the toughness may further deteriorate.

Therefore, there is a need for steel that has high strength and excellent toughness even after carbonitriding.

For example, in Patent Document 1, the steel components are C: 0.10 to 0.30%, Si: 0.30% or less, P: less than 0.03%, S: less than 0.03%, Cr: 0.7% or less, Mo: 0.01 to 0.60%, Al: 0.0010 to 0.0800%, N: 0.0010 to 0.0150%, Ti: 0.010 to 0.0800 %, B: 0.0005 to 0.0030%, the remainder being Fe and unavoidable impurities, and satisfying formulas (1) and (2). Disclosed.
F1>-1.95...Formula (1)
F2<10.0...Formula (2)
however,
F1=-1.71[C]+0.52[Si]-0.59[Mn]-0.50[Cu]-0.23[Ni]-0.18[Cr]-1.71[Mo]
F2=236.61[Si]2-31.04[Si]+33.92[Cr]2-18.48[Cr]+23.92[Si][Mn]
(In the formulas of F1 and F2, [ ] indicates the content mass % of the element in [ ].)

JP2020-29608A

The above-mentioned Patent Document 1 discloses that by specifying the amounts of Si, Cr, Mn, etc., precipitation of nitrides in carbonitriding treatment can be reduced. However, in Patent Document 1, B is contained in order to improve hardenability, and Ti is contained excessively in order to avoid B nitrides, and the reduction in toughness due to Ti nitrides is not considered.

The present invention was made in view of the above-mentioned current situation, and an object of the present invention is to provide a steel material for carbonitriding that has high strength and excellent toughness after carbonitriding treatment.

The gist of the invention is as follows.

(1) The carbonitriding steel material according to one aspect of the present invention has a chemical composition in mass%,
C: 0.10-0.30%,
Si: 0.50% or less,
Mn: 1.15-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
Cr: 0.20-0.64%,
N: 0.001 to 0.020%,
Contains Mo: 0.32 to 0.60%, and O: 0.0030% or less,
The remainder consists of Fe and impurities,
The following formula (1) is satisfied, and the Mn concentration in cementite is 1.70% by mass or more.
0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1)
However, Mn, Cr, and Mo in the above formula (1) indicate the content of the elements in mass %.
(2) A steel material for carbonitriding according to another aspect of the present invention has a chemical composition in mass%,
C: 0.10-0.30%,
Si: 0.50% or less,
Mn: 1.15-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
Cr: 0.20-0.64%,
N: 0.001 to 0.020%,
Contains Mo: 0.32 to 0.60%, and O: 0.0030% or less,
Furthermore, it contains one or more selected from the group consisting of the following groups A, B, and C,
The remainder consists of Fe and impurities,
The following formula (1) is satisfied, and the Mn concentration in cementite is 1.70% by mass or more.
0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1)
However, Mn, Cr, and Mo in the above formula (1) indicate the content in mass % of the elements.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more types selected from the group consisting of Nb: 0.010% or less, and B: 0.0015% or less [Group B]
Cu: 0.30% or less,
One or two selected from the group consisting of Ni: 0.30% or less and Sn: 0.100% or less [Group C]
Ca: 0.0050% or less,
Mg: one or two selected from the group consisting of 0.0050% or less (3) The carbonitriding steel material described in (2) above has a chemical composition containing the group A in mass%. It's okay.
(4) The carbonitriding steel material described in (2) above may have a chemical composition containing the group B in mass %.
(5) The carbonitriding steel material described in (2) above may have a chemical composition containing the C group in mass %.

According to the above aspect of the present invention, it is possible to provide a steel material for carbonitriding that has high strength and excellent toughness after carbonitriding treatment.

Hereinafter, each requirement of the steel material for carbonitriding according to this embodiment will be explained in detail. Note that in this embodiment, the steel material for carbonitriding refers to a steel material that is processed into a part shape as necessary and then subjected to carbonitriding treatment before being used (for example, used as a mechanical structural part).

<About chemical composition>
First, the chemical composition of the carbonitriding steel material according to this embodiment will be explained. The numerically limited range described below with "~" in between includes the lower limit value and the upper limit value. Numerical values indicated as "less than" or "greater than" do not include the value within the numerical range. All "%" in chemical compositions means "% by mass".

The chemical composition of the steel material for carbonitriding according to this embodiment is, in mass %, C: 0.10 to 0.30%, Si: 0.50% or less, Mn: 1.15 to 2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.080%, Cr: 0.20 to 0.64%, N: 0.001 to 0.020%, Mo: 0 .32 to 0.60%, O: 0.0030% or less, and the remainder: Fe and impurities. Each element will be explained below.

C: 0.10-0.30%
Carbon (C) increases the strength of mechanical structural parts (hereinafter sometimes simply referred to as "components") after carbonitriding treatment. In order to obtain the desired strength in the part, the C content should be 0.10% or more. Preferably it is 0.12% or more.
However, if the C content is too high, the strength of the core of the component will become too high and the toughness will decrease. Therefore, the C content is set to 0.30% or less. Preferably it is 0.28% or less.

Si: 0.50% or less Silicon (Si) has the effect of deoxidizing steel. On the other hand, Si reduces the strength of parts by forming nitrides during carbonitriding. Therefore, the Si content is set to 0.50% or less. It is preferably 0.45% or less, more preferably 0.40% or less, less than 0.40%, or 0.35% or less.
The lower limit of the Si content is not particularly limited, but may be 0.01% or more or 0.05% or more.

Mn: 1.15-2.00%
Manganese (Mn) increases the strength of parts after carbonitriding. Moreover, Mn improves the toughness of parts by being dissolved in cementite. If the Mn content is less than 1.15%, these effects cannot be obtained. Therefore, the Mn content is set to 1.15% or more. Preferably it is 1.20% or more.
On the other hand, if the Mn content exceeds 2.00%, nitrides are formed during carbonitriding treatment, thereby reducing the strength of the component. Therefore, the Mn content is set to 2.00% or less. Preferably it is 1.80% or less.

P: 0.050% or less Phosphorus (P) is unavoidably contained in steel. P tends to segregate in steel, causing a local decrease in ductility. In particular, when the P content exceeds 0.050%, the local ductility decreases significantly. Therefore, the P content is set to 0.050% or less. Preferably it is 0.030% or less.
The lower limit of the P content is not particularly limited, but may be more than 0% or 0.002% or more.

S: 0.050% or less Sulfur (S) is unavoidably contained in steel. S combines with Mn in steel to form MnS, which causes cracking during hot rolling. Therefore, the S content is set to 0.050% or less. Preferably it is 0.040% or less.
The lower limit of the S content is not particularly limited, but may be more than 0% or 0.005% or more.

Al: 0.005-0.080%
Aluminum (Al) is an effective element as a deoxidizing agent. If the Al content is less than 0.005%, the effect cannot be obtained. Therefore, the Al content is set to 0.005% or more. Preferably it is 0.010% or more.
On the other hand, if the Al content exceeds 0.080%, nitrides are formed during the carbonitriding treatment, thereby reducing the strength of the component. Therefore, the Al content is set to 0.080% or less. Preferably it is 0.060% or less.

Cr: 0.20-0.64%
Chromium (Cr) improves the hardenability of steel and increases the strength of parts after carbonitriding. This effect cannot be obtained if the Cr content is less than 0.20%. Therefore, the Cr content is set to 0.20% or more. Preferably it is 0.25% or more.
On the other hand, in the chemical composition of the steel material for carbonitriding according to the present embodiment, if the Cr content is more than 0.64%, the toughness of the component decreases by decreasing the amount of solid solution of Mn in cementite. Therefore, in this embodiment, the Cr content is set to 0.64% or less. Preferably it is less than 0.60%.

N: 0.001-0.020%
Nitrogen (N) reduces the amount of solid solution Ti by combining with Ti in the steel to form TiN. When the amount of solid solution Ti is reduced, coarse TiN will not be generated during carbonitriding treatment, and a decrease in toughness can be suppressed. In order to obtain this effect, the N content is set to 0.001% or more. Preferably it is 0.005% or more.
On the other hand, if the N content exceeds 0.020%, TiN will precipitate excessively even without carbonitriding. If TiN is excessively precipitated before carbonitriding, a large amount of TiN remains even after carbonitriding, and the toughness of the part deteriorates. Therefore, the N content is set to 0.020% or less. Preferably it is 0.018% or less.

Mo: 0.32~0.60%
Molybdenum (Mo) improves the hardenability of steel and increases the strength of parts after carbonitriding. Moreover, Mo increases the amount of Mn solid solution in cementite and improves the toughness of parts. This effect cannot be obtained if the Mo content is less than 0.32%. Therefore, the Mo content is set to 0.32% or more. Preferably it is 0.35% or more.
On the other hand, if the Mo content exceeds 0.60%, the strength of the steel material for carbonitriding becomes too high and the workability deteriorates. Therefore, the Mo content is set to 0.60% or less. Preferably it is 0.55% or less.

O: 0.0030% or less When O is contained in large amounts in steel, it forms coarse oxides that become a starting point for fracture and deteriorates the toughness of parts. Therefore, the O content is set to 0.0030% or less. The O content is preferably 0.0020% or less, more preferably 0.0015% or less.
The lower limit of the O content is not particularly limited, but may be 0% or more, more than 0%, or 0.0005% or more.

0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1)
In the steel material for carbonitriding of this embodiment, in order to preferably control the Mn concentration in cementite, it is important to control the content of alloying elements. In particular, it is important to control the Mn content, the Cr content, which has a high solid solubility in cementite and reduces the Mn concentration in cementite, and the Mo content, which stabilizes cementite. If the left side of the above formula (1) is less than 0.80% by mass, the Mn concentration in cementite may not be controlled favorably. Therefore, the left side of the above formula (1) is set to be 0.80% by mass or more.
Note that the upper limit of the left side of the above formula (1) is not particularly defined, but from the viewpoint of alloy cost, it is preferably 2.00% by mass or less. Further, the left side of the above formula (1) may be 1.70% by mass or less, 1.65% by mass or less, 1.40% by mass or less, or 1.30% by mass or less.

The remainder of the chemical composition of the steel material for carbonitriding according to this embodiment may be Fe and impurities. In this embodiment, impurities refer to impurities that are mixed in from ores as raw materials, scraps, or the manufacturing environment during industrial production of steel materials, and to the extent that they do not impede the characteristics of the steel material for carbonitriding according to this embodiment. Examples of permissible elements are listed below.

The chemical composition of the steel material for carbonitriding according to the present embodiment further includes, in place of a part of Fe, an optional element, and one or more elements selected from the group consisting of Group A, Group B, and Group C below. May contain. When the following arbitrary elements are not included, the content is 0%.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more types selected from the group consisting of Nb: 0.010% or less, and B: 0.0015% or less [Group B]
Cu: 0.30% or less,
One or two selected from the group consisting of Ni: 0.30% or less and Sn: 0.100% or less [Group C]
Ca: 0.0050% or less,
Mg: 1 type or 2 types selected from the group consisting of 0.0050% or less

Ti: 0.005% or less V: 0.010% or less Nb: 0.010% or less B: 0.0015% or less Titanium (Ti), vanadium (V), niobium (Nb), and boron (B) are carburized. During nitriding treatment, it combines with N in steel to form nitrides. If the content of these elements is too high, coarse nitrides are formed, which reduces the toughness of the core of the component. Therefore, the Ti content is 0.005% or less, the V content is 0.010% or less, the Nb content is 0.010% or less, and the B content is 0.0015% or less. Preferably, the Ti content is 0.004% or less or 0.003% or less, the V content is 0.005% or less or 0.003% or less, and the Nb content is 0.005% or less or 0. .003% or less, and the B content is 0.0010% or less or 0.0005% or less.
Since it is preferable that these elements are not contained, the content of Ti, V, Nb, and B may be 0%.

Cu: 0.30% or less Copper (Cu) increases the strength of parts. In order to obtain this effect, Cu may be included. In order to reliably obtain this effect, the Cu content is preferably 0.02% or more.
However, if the Cu content exceeds 0.30%, the toughness of the component will decrease. Therefore, the Cu content is set to 0.30% or less. Preferably it is 0.25% or less.

Ni: 0.30% or less Nickel (Ni) increases the strength of parts. Ni may be included to obtain this effect. In order to reliably obtain this effect, the Ni content is preferably 0.02% or more.
However, if the Ni content exceeds 0.30%, the strength of the component will increase and the toughness will decrease. Therefore, the Ni content is preferably 0.30% or less. Preferably it is 0.25% or less.

Sn: 0.100% or less Tin (Sn) increases the strength of parts. Sn may be contained in order to obtain this effect. In order to reliably obtain this effect, the Sn content is preferably 0.020% or more.
However, if the Sn content exceeds 0.100%, the toughness of the component will decrease. Therefore, the Sn content is preferably 0.100% or less. Preferably it is 0.050% or less.

Ca: 0.0050% or less Calcium (Ca) may be added to improve machinability during manufacturing of parts. In order to obtain this effect, the content is preferably 0.0005% or more.
However, if the Ca content exceeds 0.0050%, coarse oxides may be formed and the toughness of the component may deteriorate. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less. Ca content is preferably 0.0025% or less.

Mg: 0.0050% or less Magnesium (Mg) may be added to improve machinability during manufacturing of parts. In order to obtain this effect, the content is preferably 0.0005% or more.
However, if the Mg content exceeds 0.0050%, coarse oxides may be formed and the toughness of the component may deteriorate. Therefore, when Mg is contained, the Mg content is set to 0.0050% or less. The Mg content is preferably 0.0025% or less.

The chemical composition of the steel material for carbonitriding described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas melting-thermal conductivity method.
The content of each element is determined by rounding off the measured value based on the significant figures specified in this embodiment to the lowest digit of each element content specified in this embodiment. do.

Mn concentration in cementite: 1.70% by mass or more In the carbonitriding steel material according to the present embodiment, the Mn concentration in cementite is 1.70% by mass or more. By setting the Mn concentration in cementite to 1.70% by mass or more, high toughness can be exhibited in the core of the component after carbonitriding. The details of the mechanism by which toughness improves after carbonitriding by increasing the Mn concentration in cementite are unknown, but the present inventor believes that the solid solution Mn distribution in austenite during heating during quenching has an effect. guess.
Note that even if the Mn concentration in cementite is too high, the above effect will be saturated, so the upper limit may be set to 3.00% by mass or less.

Method for manufacturing steel for carbonitriding Next, a preferred method for manufacturing the steel for carbonitriding according to the present embodiment will be described. According to the manufacturing method described below, the carbonitriding steel material according to the present embodiment can be stably manufactured.

First, steel having the above-mentioned chemical composition is melted to produce a slab. The produced slabs are bloomed and rolled to produce steel slabs. A steel material for carbonitriding is obtained by hot rolling the obtained steel piece. At this time, in order to sufficiently dissolve Mn in the austenite, the heating temperature of the steel piece is preferably 1050° C. or higher, and the heating time is preferably 1 hour or longer. Further, in order to ensure sufficient time for Mn to concentrate into cementite, the cementite transformation temperature is preferably high. Therefore, it is preferable that the finishing temperature during hot rolling is 850°C or higher, the area reduction rate in finishing is 30% or higher, and the average cooling rate after finishing is 0.5°C/s or higher. .

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

Molten steel was produced by performing primary refining and secondary refining, and slabs having the chemical composition shown in Table 1 were obtained by continuous casting. The produced slabs were subjected to blooming rolling to produce steel slabs. The obtained steel slab was heated to 1150°C for 1.5 hours, the finishing temperature of hot rolling was 900°C, the area reduction rate in finishing was 40%, and the average cooling from after finishing to 300°C. A round steel bar with a diameter of 60 mm was obtained at a speed of 1.0° C./s.

On the cross section parallel to the longitudinal direction including the center line of the obtained round steel bar, a microstructure observation tube was installed so that a depth of 1/2 of the radius of the cross section of the steel bar (15 mm depth from the circumferential surface) could be observed. A test piece was taken. When the structure of this test piece was observed under an optical microscope by nital corrosion, it was found that it was entirely a ferrite-pearlite structure. Furthermore, the microstructure was observed using a scanning electron microscope (SEM), and the amount of Mn in the cementite was quantitatively analyzed using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. Observation by SEM involves observing 5 fields of view at a magnification of 2000 times, identifying white parts observed in the pearlite structure (layered structure of ferrite and cementite) in the secondary electron image observation field as cementite, and identifying the cementite. The Mn concentration in cementite was obtained by performing EDS analysis. The accelerating voltage during EDS analysis was 20 kV, one point in each field of view was measured, and the average value of the Mn concentration in cementite obtained from the measurements at five points in total was taken as the measured value.
The measured values obtained are shown in Table 1.

In addition, a round steel bar was processed into a test piece with a diameter of 30 mm, and an austenitizing treatment was performed in which the test piece was heated at 800° C. for 60 minutes. Thereafter, quenching was performed by immersing it in oil at room temperature. Thereafter, a round bar test piece was obtained by performing tempering by heating at 150° C. for 60 minutes. This round bar test piece simulates the core of a part when carbonitriding is performed on steel. A tensile test piece and an impact test piece were taken from the obtained round steel bar centered at the 1/2 radius position. The tensile test piece was a No. 14A rod-shaped test piece having a parallel part length of 35 mm, a parallel part diameter of 5 mm, and a grip part diameter of 10 mm, in accordance with JIS Z 2241:2011. The average tensile strength TS (GPa) was determined by conducting a tensile test based on JIS Z 2241:2011 using five of these rod-shaped test pieces.
The impact test piece was a U-notch test piece having a length of 55 mm, a cross section of 10 mm square, a notch depth of 2 mm, and a notch bottom radius of 1 mm, in accordance with JIS Z 2242:2018. The notch was machined on the surface close to the surface side of the round bar. Using five of these U-notch test pieces, a Charpy impact test was conducted at room temperature in accordance with JIS Z 2242:2018 to determine the average absorbed energy (J/cm ² ).
Table 2 shows the test results.

If the obtained absorbed energy is 100 J/cm ² or more and the product of absorbed energy and tensile strength TS is 115 J/cm ² ·GPa or more, high strength and excellent toughness are obtained after carbonitriding treatment. was determined to have been obtained.
If either one of the conditions was not satisfied, it was determined that high strength and excellent toughness could not be obtained after the carbonitriding treatment.

Looking at Tables 1 and 2, it can be seen that the carbonitriding steel materials according to the examples of the present invention had high strength and excellent toughness after the carbonitriding treatment. On the other hand, it can be seen that high strength and excellent toughness were not obtained with the steel material for carbonitriding according to the comparative example.

Claims (5)

The chemical composition is in mass%,
C: 0.10-0.30%,
Si: 0.50% or less,
Mn: 1.15-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
Cr: 0.20-0.64%,
N: 0.001 to 0.020%,
Contains Mo: 0.32 to 0.60%, and O: 0.0030% or less,
The remainder consists of Fe and impurities,
A steel material for carbonitriding, which satisfies the following formula (1) and has a Mn concentration in cementite of 1.70% by mass or more.
0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1)
However, Mn, Cr, and Mo in the above formula (1) indicate the content of the elements in mass %. The chemical composition is in mass%,
C: 0.10-0.30%,
Si: 0.50% or less,
Mn: 1.15-2.00%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.080%,
Cr: 0.20-0.64%,
N: 0.001 to 0.020%,
Contains Mo: 0.32 to 0.60% and O: 0.0030% or less,
Furthermore, it contains one or more selected from the group consisting of the following groups A, B, and C,
The remainder consists of Fe and impurities,
A steel material for carbonitriding, which satisfies the following formula (1) and has a Mn concentration in cementite of 1.70% by mass or more.
0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1)
However, Mn, Cr, and Mo in the above formula (1) indicate the content in mass % of the elements.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more types selected from the group consisting of Nb: 0.010% or less, and B: 0.0015% or less [Group B]
Cu: 0.30% or less,
One or two selected from the group consisting of Ni: 0.30% or less and Sn: 0.100% or less [Group C]
Ca: 0.0050% or less,
Mg: 1 type or 2 types selected from the group consisting of 0.0050% or less The steel material for carbonitriding according to claim 2, having a chemical composition containing the group A in mass %. The steel material for carbonitriding according to claim 2, having a chemical composition containing the group B in mass %. The steel material for carbonitriding according to claim 2, having a chemical composition containing the C group in mass %.