JP2024013082A - Case-hardening steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case-hardening steel having high strength and superior toughness after carburizing treatment.

SOLUTION: A case-hardening steel is adopted that includes, in mass%, Cr: 1.05-1.50%, Mo: 0.03-0.15%, and Ni: 0.03-0.19%, and satisfies the following (1), with the Cr content in cementite being 1.30 mass% or more. The formula (1) is: 2.20≤1.5×Cr+4.5×Mo≤2.90 (1), where, each element symbol denotes the content of the element in mass%.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to case hardening steel.

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. Carburizing is a process in which a part is heated in an austenite temperature range in a carburizing atmosphere, then quenched, and then further tempered. 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 quenched and tempered.

In response to the recent demand for high fatigue strength, alloying elements are added to increase the strength of the core of parts. However, adding alloying elements to increase the strength of the core of the part reduces the toughness of the part. Therefore, there is a need for case hardening steel that has a high balance of strength and toughness.

For example, Patent Document 1 describes a case-hardening steel for use in carburizing to obtain mechanical structural parts, which includes steel components in mass %, C: 0.2 to 0.3%, Si: 0.6% or less, Mn: 0.95 to 2.2%, P: 0.03% or less, S: 0.03% or less, Cr: 0.1 to 1.8, Al: 0.06% or less , N: 0.02% or less and O: 0.003% or less, and a case hardening steel that satisfies the following formula (1) and the following formula (2) has been proposed.
-163×[C]+43.1×[Si]-55.2×[Mn]+32.6×[Cu]-30.0×[Ni]-47.8×[Cr]+104×[Mo]+412 ×[V]+677≦540…(1)
40.4×[C]+1.31×[Si]+18.7×[Mn]+8.37×[Cu]+5.33×[Ni]+5.57×[Cr]+11.8×[Mo]- 51.1×[V]-17.8≧15 (2)
However, the element symbol in [ ] indicates the content (mass%).

Japanese Patent Application Publication No. 2012-1774

Patent Document 1 discloses that case hardening steel with extremely small heat treatment strain can be realized by appropriately controlling the components and transformation start temperature. However, Patent Document 1 does not particularly consider strength and toughness.

The present invention has been made in view of the above-mentioned current situation, and an object of the present invention is to obtain case hardening steel having high strength and excellent toughness after carburizing treatment.

The gist of the invention is as follows.

(1) The case hardening steel according to one aspect of the present invention has a chemical composition in mass %,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19%, and O: 0.0030% or less, with the remainder consisting of Fe and impurities,
A case hardening steel that satisfies the following (1) and has a Cr concentration in cementite of 1.30% by mass or more.
2.20≦1.5×Cr+4.5×Mo≦2.90…(1)
However, the element symbol in the above formula (1) indicates the content in mass % of each element.
(2) The case hardening steel according to another aspect of the present invention has a chemical composition in mass %,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Ni: 0.03 to 0.19%, and O: 0.0030% or less, and further 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 (1) is satisfied, and the Cr concentration in cementite is 1.30% by mass or more.
2.20≦1.5×Cr+4.5×Mo≦2.90…(1)
However, the element symbol in the above formula (1) indicates the content in mass % of each element.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more types selected from the group consisting of Nb: 0.005% or less, and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
One or two types selected from the group consisting of Ca: 0.0050% or less, and Mg: 0.0050% or less (3) The case hardening steel described in (2) above, in mass %, It may have a chemical composition containing.
(4) The case hardening steel described in (2) above may have a chemical composition containing the group B in mass %.
(5) The case hardening steel 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 case hardening steel having high strength and excellent toughness after carburizing treatment.

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

<About chemical composition>
First, the chemical composition of the case hardening steel 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 case hardening steel according to the present embodiment has a chemical composition in mass %: C: 0.17 to 0.35%, Si: 0.28 to 0.60%, Mn: 1.03 to 1.45%. , P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 1.05 to 1.50%, N: 0.020% or less, Mo: 0 .03 to 0.15%, Ni: 0.03 to 0.19%, O: 0.0030% or less, and the remainder: Fe and impurities. Each element will be explained below.

C: 0.17-0.35%
Carbon (C) increases the strength of mechanical structural parts (hereinafter sometimes simply referred to as "components") after carburizing treatment. In order to obtain the desired strength in the part, the C content should be 0.17% or more. The C content is preferably 0.19% or more.
However, if the C content exceeds 0.35%, the strength of the core of the component becomes too high and the toughness deteriorates. Therefore, the C content is set to 0.35% or less. The content of C is preferably 0.32% or less.

Si: 0.28-0.60%
Silicon (Si) improves the hardenability of case hardening steel. In order to improve the hardenability of case hardening steel and obtain desired strength in parts, the Si content is set to 0.28% or more. The Si content is preferably 0.30% or more.
However, if the Si content exceeds 0.60%, the toughness of the component deteriorates. Therefore, the Si content is set to 0.60% or less. The Si content is preferably 0.55% or less.

Mn: 1.03-1.45%
Manganese (Mn) increases the strength of parts. If the Mn content is less than 1.03%, desired strength cannot be obtained in the component. Therefore, the Mn content is set to 1.03% or more. The Mn content is preferably 1.10% or more.
On the other hand, if the Mn content exceeds 1.45%, the toughness of the component will deteriorate. Therefore, the Mn content is set to 1.45% or less. The Mn content is preferably 1.40% 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%, local ductility decreases significantly. Therefore, the P content is set to 0.050% or less. The P content is preferably 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 may cause cracking during hot forging. When the S content exceeds 0.050%, cracking becomes noticeable. Therefore, the S content is set to 0.050% or less. The S content is preferably 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: 0.005-0.050%
Aluminum (Al) is an effective element as a deoxidizing agent. If the Al content is less than 0.005%, deoxidation will be insufficient. Therefore, the Al content is set to 0.005% or more. Al content is preferably 0.010% or more.
On the other hand, if the Al content exceeds 0.050%, coarse oxides are formed and the toughness of the component deteriorates. Therefore, the Al content is set to 0.050% or less. Al content is preferably 0.040% or less.

Cr: 1.05-1.50%
Chromium (Cr) improves the hardenability of case hardening steel, increases the strength of the core of the part after carburizing treatment, and improves the toughness of the core of the part by stabilizing it by dissolving in carbide. If the Cr content is less than 1.05%, the above effects cannot be obtained. Therefore, the Cr content is set to 1.05% or more. The Cr content is preferably 1.10% or more.
On the other hand, if the Cr content exceeds 1.50%, not only the solid solution in the carbide becomes saturated, but also the toughness of the part deteriorates. Therefore, the Cr content is set to 1.50% or less. The Cr content is preferably 1.30% or less.

N: 0.020% or less Nitrogen (N) is unavoidably contained in steel. Additionally, nitrogen may combine with Ti in the steel to form TiN, thereby degrading the toughness of the core of the component. If the N content exceeds 0.020%, the toughness of the core of the component will deteriorate significantly. Therefore, the N content is set to 0.020% or less. The N content is preferably 0.018% or less, more preferably 0.015% or less.
The lower limit of the N content is not particularly limited, but may be more than 0% or 0.005% or more.

Mo: 0.03~0.15%
Molybdenum (Mo) improves the hardenability of case-hardening steel, increases the strength of the core of parts after carburizing, and improves the toughness of the core of parts by stabilizing it by dissolving in carbide. If the Mo content is less than 0.03%, the above effects cannot be obtained. Therefore, the Mo content is set to 0.03% or more. Mo content is preferably 0.05% or more.
On the other hand, if the Mo content exceeds 0.15%, not only the solid solution in the carbide becomes saturated, but also the toughness of the part deteriorates. Therefore, the Mo content is set to 0.15% or less. Mo content is preferably 0.13% or less.

Ni: 0.03-0.19%
Nickel (Ni) increases the toughness of parts. This effect cannot be obtained if the Ni content is less than 0.03%. Therefore, the Ni content is set to 0.03% or more. Ni content is preferably 0.05% or more.
On the other hand, if the Ni content exceeds 0.19%, the toughness of the component will deteriorate. Therefore, the Ni content is set to 0.19% or less. Ni content is preferably 0.18% 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.

2.20≦1.5×Cr+4.5×Mo≦2.90…(1)
In the steel material for carbonitriding of this embodiment, in order to preferably control the Cr concentration in cementite, it is important to control the content of alloying elements. In particular, it is important to control the Cr content and the Mo content that stabilizes cementite.
If the value of the middle side of the above formula (1) is 2.20 or more, the Cr concentration in cementite can be preferably controlled. As a result, high toughness is obtained in the part. Therefore, the value of the middle side of equation (1) is set to 2.20 or more. Preferably, it is 2.30 or more or 2.35 or more.
On the other hand, if the value of the middle side of equation (1) exceeds 2.90, not only the above effect is saturated, but also the toughness of the part deteriorates due to solid solution strengthening of Cr and Mo. Therefore, the value of the middle side of equation (1) is set to 2.90 or less. Preferably, it is 2.80 or less or 2.70 or less.

The remainder of the chemical composition of the case hardened steel according to this embodiment may be Fe and impurities. In this embodiment, impurities refer to impurities that are mixed in from ores used as raw materials, scrap, or the manufacturing environment during industrial production of steel materials, and are allowed as long as they do not impede the characteristics of the case-hardened steel according to this embodiment. Examples of elements that are

The chemical composition of the case hardening steel according to the present embodiment further includes one or more selected from the group consisting of Group A, Group B, and Group C below as an optional element in place of a part of Fe. May be contained. 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: 0.005% or less, and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less

Ti: 0.005% or less Titanium (Ti) combines with N in steel to form TiN, but if the Ti content exceeds 0.005%, coarse TiN will be formed and the core of the part will be damaged. It may deteriorate the toughness of the parts. Therefore, the Ti content is set to 0.005% or less.
Since it is preferable that Ti is not contained, the Ti content may be 0%.

V: 0.010% or less V is an element that improves the strength of steel materials through solid solution strengthening. In order to obtain this effect, V may be contained as necessary. However, if the V content exceeds 0.010%, a large amount of carbonitrides may precipitate and the toughness of the part may deteriorate. Therefore, the V content is preferably 0.010% or less.
The V content may be 0%.

Nb: 0.005% or less Niobium (Nb) combines with N in steel to form NbN, but if the Nb content exceeds 0.005%, coarse NbN is formed and the core of the part It may deteriorate the toughness of the parts. Therefore, the Nb content is set to 0.005% or less.
Since it is preferable that Nb is not contained, the Nb content may be 0%.

B: 0.0015% or less B is an element that combines with N in steel to form nitrides during induction hardening. If the B content exceeds 0.0015%, coarse nitrides may be formed and the toughness of the parts after induction hardening may be deteriorated. Therefore, the B content is preferably 0.0015% or less.
Since it is preferable that B is not contained, the B content may be 0%.

Cu: 0.50% or less Copper (Cu) increases the strength of parts. In order to obtain this effect, Cu may be included as necessary. In order to reliably obtain the above effects, the Cu content is preferably 0.02% or more.
However, if the Cu content exceeds 0.50%, the toughness of the component may deteriorate. Therefore, when Cu is contained, the Cu content is 0.50% or less. The Cu content is preferably 0.45% or less.

Sn: 0.100% or less Sn is an element that suppresses coarsening of crystal grains and improves the strength of steel materials. In order to obtain this effect, Sn may be contained as necessary. However, if the Sn content exceeds 0.100%, the steel becomes brittle and easily breaks during hot rolling. Therefore, the Sn content is preferably 0.100% or less.
The Sn content may be 0%.

Ca: 0.0050% or less Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the formability of steel materials. In order to obtain this effect, Ca may be contained as necessary. However, even if the Ca content exceeds 0.0050%, the above effects are saturated. Therefore, the Ca content is preferably 0.0050% or less.
The Ca content may be 0%.

Mg: 0.0050% or less Mg has the effect of improving the formability of the steel material by adjusting the shape of inclusions in the steel to a preferable shape. In order to obtain this effect, Mg may be included as necessary. However, when the Mg content exceeds 0.0050%, inclusions are excessively generated in the steel, which may actually reduce the formability of the steel material. Therefore, the Mg content is preferably 0.0050% or less.
The Mg content may be 0%.

The chemical composition of the above-mentioned case hardened steel 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.
In addition, the content of each element is determined by rounding off the fraction of the measured value based on the significant figures specified in this embodiment, and calculates the value to the smallest digit of the content of each element specified in this embodiment. do.

Cr concentration in cementite: 1.30% by mass or more In the carbonitriding steel material according to the present embodiment, the Cr concentration in cementite is 1.30% by mass or more. By setting the Cr concentration in cementite to 1.30% by mass or more, high toughness can be exhibited in the core of the part after carbonitriding. Although the details of the mechanism by which toughness improves after carbonitriding by increasing the Cr concentration in cementite are unknown, the present inventor believes that the distribution of solid solution Cr in austenite during heating during quenching has an effect. guess.
Note that even if the Cr 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 case-hardened steel Next, a method for manufacturing case-hardened steel according to the present embodiment will be described. Although the manufacturing method is not particularly limited, examples include the following method.
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 Cr in the austenite, the heating temperature of the steel slab is preferably 1050° C. or higher, and the heating time is preferably 1 hour or longer. Further, in order to ensure sufficient time for Cr 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.

A 150 kg ingot having the chemical composition shown in Tables 1A and 1B was melted using a vacuum melting furnace. The obtained ingot was heated at 1200°C for 1.5 hours, then hot forged at a finish processing temperature of 900°C with an area reduction rate of 40%, and average cooling down to 300°C after finishing processing. A round steel bar with a diameter of 35 mm was obtained at a speed of 1.0° C./s.

Microstructure observation was carried out so that a depth of 1/2 of the cross-sectional radius of the steel bar (8.75 mm depth from the circumferential surface) on a cross section parallel to the longitudinal direction including the center line of the obtained round steel bar 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 Cr 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 Cr concentration in cementite was obtained by performing EDS analysis. The accelerating voltage during EDS analysis was 20 kV, one point in each visual field was measured, and the average value of the Cr concentration in cementite obtained from the measurements at five points in total was taken as the measured value.
The measurement times obtained are shown in Table 1A and Table 1B.

A round steel bar was processed into a test piece with a diameter of 30 mm, and an austenitizing treatment was performed by heating it at 850° C. for 30 minutes. Thereafter, quenching was performed by immersing it in oil at room temperature, followed by tempering by heating at 160° C. for 60 minutes to obtain a round bar test piece. This round bar test piece simulates the core of a part when case hardening steel is carburized.

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. Five rod-shaped test pieces were used to perform a tensile test in accordance with JIS Z 2241:2011 to determine the average tensile strength TS (GPa).
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.

The obtained absorbed energy is 100 J/cm ² or more, the obtained tensile strength is 1.05 GPa or more, and the product of the absorbed energy and the tensile strength TS is 125 J/cm ² ·GPa or more. It was determined that high strength and excellent toughness were obtained after carburizing treatment.
If either one of the conditions was not satisfied, it was determined that high strength and excellent toughness could not be obtained after the carburizing treatment.

Looking at Tables 1A, 1B, and 2, it can be seen that the case hardened steels according to the examples of the present invention had high strength and excellent toughness after carburizing treatment. On the other hand, it can be seen that high strength and excellent toughness were not obtained with the case hardened steel according to the comparative example.

Claims (5)

The chemical composition is in mass%,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Contains Ni: 0.03 to 0.19%, and O: 0.0030% or less, with the remainder consisting of Fe and impurities,
A case hardening steel that satisfies the following (1) and has a Cr concentration in cementite of 1.30% by mass or more.
2.20≦1.5×Cr+4.5×Mo≦2.90…(1)
However, the element symbol in the above formula (1) indicates the content in mass % of each element. The chemical composition is in mass%,
C: 0.17-0.35%,
Si: 0.28-0.60%,
Mn: 1.03 to 1.45%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.005-0.050%,
Cr: 1.05-1.50%,
N: 0.020% or less,
Mo: 0.03-0.15%,
Ni: 0.03 to 0.19%, and O: 0.0030% or less, and further 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 case hardening steel that satisfies the following (1) and has a Cr concentration in cementite of 1.30% by mass or more.
2.20≦1.5×Cr+4.5×Mo≦2.90…(1)
However, the element symbol in the above formula (1) indicates the content in mass % of each element.
[Group A]
Ti: 0.005% or less,
V: 0.010% or less,
One or more types selected from the group consisting of Nb: 0.005% or less, and B: 0.0015% or less [Group B]
One or two selected from the group consisting of Cu: 0.50% or less and Sn: 0.100% or less [Group C]
One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less The case hardening steel according to claim 2, having a chemical composition containing the group A in mass %. The case hardening steel according to claim 2, having a chemical composition containing the group B in mass %. The case hardening steel according to claim 2, having a chemical composition containing the C group in mass %.